Abstract:

A polylactic acid monofilament includes a linear polylactic acid with a
relative viscosity ηrel of in the range of 2.7 to 4.5, prepared from
lactic acid monomers wherein at least 95 mol % of the lactic acid is an
L-isomer, and wherein the resin contains 0 to 30 ppm of Sn and 0 to 0.5%
by weight of residual monomer.

Claims:

1. A polylactic acid monofilament comprising a linear polylactic acid with
a relative viscosity ηrel in the range of 2.7 to 4.5, prepared from
lactic acid monomers wherein at least 95 mol % of the lactic acid is an
L-isomer, and wherein the resin contains 0 to 30 ppm of Sn and 0 to 0.5%
by weight of residual monomer.

2. The polylactic acid monofilament according to claim 1, having a tensile
strength of 3.5 cN/dtex or more, an elongation of 40.0% or less, a
contraction ratio in boiling water of 10.0% or less and a birefringence,
Δn, of 0.0250 or more.

3. A process for producing a polylactic acid monofilament with a
polylactic acid resin having a relative viscosity wel in the range of 2.7
to 4.5, prepared from lactic acid monomers wherein at least 95 mol % of
the lactic acid is an L-isomer, and wherein the resin contains 0 to 30
ppm of Sn and 0 to 0.5% by weight of residual monomer, the process
comprising spinning the resin at a temperature of 220.degree. C. to
250.degree. C., drawing monofilament at a draw magnification factor of
6.0 or more at a temperature of 70.degree. C. to 100.degree. C., and
heat-treating drawn monofilament at a temperature of 100.degree. C. to
150.degree. C.

Description:

RELATED APPLICATIONS

[0001]This application is a divisional of U.S. patent application Ser. No.
12/199,245, filed on Sep. 26, 2008, which is a divisional under 35 USC
§120 of prior U.S. patent application Ser. No. 10/018,732, filed on
Mar. 8, 2002 as a national phase entry of International Application No.
PCT/JP00/04000, filed on Jun. 19, 2000, which claims the benefit of
Japanese Patent Application Nos. 11/172,414, filed on Jun. 18, 1999;
11/205,836, filed on Jul. 21, 1999; 11/205,838, filed on Jul. 21, 1999;
11/210,370, filed on Jul. 26, 1999; 11/216,585, filed on Jul. 30, 1999;
11/259,914, filed on Sep. 14, 1999; 11/264,727, filed on Sep. 20, 1999;
11/273,086, filed on Sep. 27, 1999; and 2000/609, filed on Jan. 6, 2000,
all of which are incorporated herein by reference in their entireties.

TECHNICAL FIELD

[0002]This disclosure relates to a resin mainly comprising polylactic acid
and textile products using the resin as a starting material, and
processes for producing the textile products.

BACKGROUND

[0003]The most widely used textile materials today include synthetic
resins such as poly-esters represented by polyethylene terephthalate and
polyamides represented by 6-nylon and 66-nylon.

[0004]While these synthetic resins are advantageous in their capability of
cheap mass production, they involve some problems related to their
disposal. The textile made of such synthetic resins can be hardly
decomposed in the natural environment, and high heat of combustion is
generated by incineration.

[0005]Under these situations, use of biodegradable synthetic resins such
as polycaprolactone and polylactic acid for textiles have been proposed.
Although these resins are excellent in bio-degradability, they are still
not suitable for practical applications as compared with non-degradable
synthetic resins such as polyethylene terephthalate and nylon that have
been widely used.

[0006]These problems are poor process throughput during the producing
process (spinning, drawing, false twisting and the like), inferior
properties such as tensile strength and elongation percentage of the
textile products obtained as compared with conventional synthetic fibers.

SUMMARY

[0007]We surveyed the physical and chemical properties of polylactic acid,
and have investigated polylactic acid resins particularly suitable for
use in the textile products. We found polylactic acid textile products
being excellent in productivity and having favorable properties by using
polylactic acid having selected properties, and a process for producing
the textile products. We thus provide practically acceptable textile
products comprising polylactic acid having excellent properties for use
in textiles with high productivity.

[0008]We provide a polylactic acid resin mainly comprising linear
polylactic acid comprising 95 mol % or more of the L-isomer and
containing 0 or 30 ppm or less of tin(Sn) and 0 or 0.5% by weight or less
of monomer content with a relative viscosity ηrel of 2.7 to 3.9, and
a polylactic acid resin mainly comprising linear polylactic acid
comprising 95 mol % or more of the L-isomer and containing 0 or 30 ppm or
less of Sn and 0 or 0.5% by weight or less of monomer content with a
weight average molecular weight Mw of 120,000 to 220,000 and number
average molecular weight Mn of 60,000 to 110,000. We also provide a
textile product mainly using the polylactic acid resin as a starting
material.

[0011]The polylactic acid resin, fiber thereof, and the process for
producing them will be described first.

[0012]The polylactic acid resin include (1) a polylactic acid resin mainly
comprising linear polylactic acid comprising 95 mol % or more of the
L-isomer and containing 0 or 30 ppm or less of Sn and 0 or 0.5% by weight
or less of monomer content with a relative viscosity ηrel of 2.7 to
3.9, and (2) a polylactic acid resin mainly comprising linear polylactic
acid comprising 95 mol % or more of the L-isomer and containing 0 or 30
ppm or less of Sn and 0 or 0.5% by weight or less of monomer content with
a weight average molecular weight Mw of 120,000 to 220,000 and number
average molecular weight Mn of 60,000 to 110,000. The polylactic acid
fiber and the producing process thereof comprise the following elements:
(3) a polylactic acid fiber comprising the polylactic acid resin in (1)
or (2) above; and (4) a process for producing the polylactic acid fiber
by melt-spinning using polylactic acid in (1) or (2).

[0013]Polylactic acid to be used herein has a linear structure, or
substantially has no branched structure. A small amount of branching
agent has been added during polymerization of polylactic acid to improve
melt viscosity and degree of polymerization in the former proposal.
However, we confirmed that the branched structure of the starting resin
material for producing the polylactic acid fiber has a far more negative
effect on spinning work efficiency as compared with production of
conventional polyester fibers. In other words, even a small amount of the
branched structure adversely affects spinning work efficiency of
polylactic acid, besides the fiber obtained has a low tensile strength.

[0014]For excluding the branched structure, it is recommended that
chemicals that causes branched structures in the polymer material, for
example three valent or four valent alcohols and carboxylic acids, are
not used at all. When these chemicals are forced to use for some other
reasons, the amount of use should be restricted within a range as small
as possible so that spinning work efficiency is not adversely affected.

[0015]Although polylactic acid used herein is derived from a starting
material such as L-lactic acid or D-lactic acid, or L-lactide or
D-lactide as a dimer of lactic acid, or mesolactide, it is essential that
the proportion of the L-isomer is 95 mol % or more. This is because
increased proportion of the D-isomer makes the polymer amorphous and
crystal orientation is not advanced in the spinning and drawing process,
thereby deteriorating the properties of the fiber obtained. In
particular, the tensile strength remarkably decreases with excess
contraction ratio in boiling water to make the fiber to be practically
inapplicable.

[0016]Polylactic acid to be used herein is required to contain 0 or 30 ppm
or less, preferably 0 or 20 ppm or less, of Sn content in the polymer.
While the Sn content based catalyst is used as a polymerization catalyst
of polylactic acid, a content of more than 30 ppm causes depolymerization
during the spinning process to allow the filtration pressure at the
nozzle to increase in a short period of time, thereby remarkably
decreasing spinning work efficiency.

[0017]For decreasing the Sn content, the amount of use for polymerization
may be decreased, or chips may be washed with an appropriate solvent.

[0018]The polylactic acid contains 0.5% by weight, preferably 0.3% by
weight or less and particularly 0 or 0.2% by weight or less, of monomers.
The monomer as defined herein is referred to the component having a
molecular weight of 1,000 or less as calculated from a GPC assay. A
content of the monomer of more than 0.5% by weight causes remarkable
decrease of work efficiency, because heat resistance of polylactic acid
decreases due to heat decomposition of the monomer component.

[0019]For reducing the monomer content in polylactic acid, unreacted
monomers are removed by evacuation of the reaction vessel immediately
before completing the polymerization reaction, polymerized chips are
washed with an appropriate solvent, or polylactic acid is produced by a
solid phase polymerization.

[0020]The polylactic acid preferably has a weight average molecular weight
Mw of 120,000 to 220,000 and number average molecular weight Mn of 60,000
to 110,000. While the molecular weight in this range afford excellent
spinning ability and sufficient tensile strength, the molecular weight
out of this range causes large decrease of the molecular weight during
sinning to fail in obtaining a sufficient tensile strength.

[0021]The polylactic acid has a relative viscosity ηrel of 2.7 to 3.9.
The relative viscosity of lower than this range causes to reduce heat
resistance of the polymer and to fail in obtaining a sufficient strength,
while the relative viscosity of higher than this range requires an
elevated spinning temperature to cause heat-degradation during the
spinning process.

[0022]The relative viscosity having a lower reduction ratio during the
spinning process is favorable and the preferable reduction ratio of
relative viscosity is 7% or less for spinning multifilaments. A reduction
ratio of 7% or less substantially results in no decomposition of the
polymer during spinning, give rise to good spinning ability without
arising broken fibers during spinning, and enabling particularly high
tensile strength in the drawing process.

[0023]It is preferable for practical production that the fiber produced
has a tensile strength of 3.5 cN/dtex or more.

[0025]The fiber can be obtained by melt-spinning process known in the art.

[0026]A biodegradable fiber excellent in work efficiency and properties of
the textile may be obtained by producing the polylactic acid fiber using
the resin. According to the process, the polylactic acid fiber having
physical properties such as tensile strength, drawing ratio and
contraction ratio in boiling water comparable to conventional polyester
and nylon fibers can be obtained, wherein the fiber is excellent in heat
resistance without decreasing spinning ability, the spinning nozzle has a
sufficiently long service life, and the fibers are free from breakage and
fluffs.

[0027]This disclosure will be described in more detail with reference to
examples. Analysis of the properties of the polymer will be described
first.

Molecular Weight/Monomer Content

[0028]Samples were dissolved in chloroform in a concentration of 10 mg/mL,
and Mw and Mn were measured by the GPC assay using Waters LC Model I Plus
equipped with a R1 detector. Polystyrene was used as a standard substance
of the molecular weight.

[0029]The proportions of the monomer in the polymer were calculated from
the proportion of the component having a molecular weight of 1,000 or
less.

Relative Viscosity

[0030]The samples were dissolved in a mixed solvent of
phenol/tetrachloroethane=60/40 (in weight ratio) in a concentration of 1
g/dL, and the relative viscosity was measured at 20° C. using a
Ubberohde viscosity tube.

Sn Content

[0031]The sample (0.5 g) was ashing by a wet process using sulfuric
acid/nitric acid. The ashing sample was diluted with water to give a 50
mL sample solution, and the Sn content was measured using an ICP emission
spectrometer SRS 1500VR made by Seiko Instruments Inc.

Heat Stability

[0032]The temperature at a mass reduction of the polymer of 5% was
measured as TG (5%) using Seiko Instruments Inc TG/DTA 220U.

[0033]Spinning work efficiency and fiber properties were measured and
evaluated as follows.

Evaluation of Spinning Ability--1

[0034]A 7-days' continuous spinning was performed by melt spinning.
Incidence of broken fibers were evaluated in three steps (A, B and C)
below: [0035]A: zero time of broken fiber in 7 days; [0036]B: one to
two times of broken fiber in 7 days; and [0037]C: three or more times of
broken fiber in 7 days.

Evaluation of Spinning Ability--2

[0038]Service life of the spinning nozzle was evaluated in terms of days
when the spinning nozzle was forced to change by increment of filtration
pressure during the 7-days' continuous spinning.

Evaluation of Spinning Ability--3

[0039]Incidence of broken fibers in the drawing process was evaluated in
three steps of A, B and C: [0040]A: zero time of broken fiber in 7
days; [0041]B: one to two times of broken fiber in 7 days; and [0042]C:
three or more times of broken fiber in 7 days.

Measurements of Tensile Strength and Elongation Percentage

[0043]Using a tensile strength tester manufactured by Shimadzu Co., a
tensile test was per-formed at a speed of 20 cm/min using a sample with a
length of 20 cm, and the tensile strength and elongation percentage was
measured from the ultimate strength and ultimate elongation percentage,
respectively.

Contraction Ratio in Boiling Water

[0044]A 200 mg weight was hanged to a sample with an initial length of 50
cm, and the sample was immersed in boiling water for 15 minutes followed
by drying in the air for 5 minutes. The contraction ratio in boiling
water was determined by the following equation:

[0045]Incidence of fluffs after reeling the drawn fiber was evaluated by
the following two steps (∘ and x): [0046]∘: no
incidence of fluffs; and [0047]x: incidence of fluffs.

Productivity of Filament

[0048]Total evaluations of the filament was made in three steps of A, B
and C by considering the evaluation of spinning ability 1, 2 and 3, and
incidence of fluffs: [0049]A: very good [0050]B: good [0051]C: poor.

Rate of Decrease of Viscosity During Spinning

[0052]The relative viscosity (ηrel) of the filament extruded out of
the spinning nozzle was measured, and the rate of decrease of viscosity
during spinning was determined from the following equation. The residence
time of the molten polymer in this example was about 10 minutes.

The rate of decrease of viscosity during spinning(%)=((relative viscosity
of the polymer-relative viscosity of the filament)/relative viscosity of
the polymer)×100.

Polymerization of the Polymer

[0053]L-lactide or D-lactide as a starting material was polymerized to
polylactic acid using tin octylate as a polymerization catalyst by
conventional polymerizing step. Polymerization was also carried out by
adding 0.1 mol % of trimellitic acid as a cross-link agent (Comparative
Example 10). While the polymer obtained was subsequently subjected to
solid state polymerization at 135° C. to reduce the amount of the
residual monomers, the solid state polymerization was omitted in a part
of the samples for comparison.

Spinning

[0054]Filaments of 84 dtex/24f were obtained by a conventional filament
process of spinning and drawing by extruding the molten resin in the air
through a spinning nozzle with a spinning hole diameter of 0.25 mm and
number of spinning holes of 24. The spinning test was continued for 7
days to evaluate spinning ability, service life of the nozzle and
incidence of fluffs during drawing.

Examples 1-1 to 1-2, and Comparative Examples 1-1 to 1-5

[0055]Table 1-1 shows the changes of spinning ability, service life of the
nozzle and incidence of fluffs during drawing when the content of Sn in
the polymer is changed, and the results of the quality of the fiber.

[0056]In Comparative Examples 1-1 to 1-3, the polymer had been
depolymerized during spinning due to particularly large content of Sn
(the amount of the residual catalyst). Consequently, the viscosity was
largely decreased during the spinning step to make it very difficult to
spin. In addition, the service life of the nozzle was a short as one day,
quite large number of fluffs had generated during the spinning step due
to large rate of decrease of viscosity during the drawing step, and the
fiber obtained had a quite poor tensile strength of 2.6 cN/dtex or less
to make it impossible to use the fiber for practical purposes.

[0057]While the rate of decrease of viscosity during spinning was improved
to 17.6% in Comparative Example 1-4, the service life of the nozzle was
as short as three days. Although incidence of fluffs during drawing was
also improved, the fiber was inappropriate for practical uses since a
practical tensile strength of the fiber of 3.5 cN/dtex was not attained.

[0058]The service life of the nozzle was increased to six days and the
tensile strength of the fiber satisfied the practical level of 3.5
cN/dtex or more in Comparative Example 1-5, since the rate of decrease of
viscosity during spinning was improved to 12.3%. However, improvement of
incidence of fluffs was yet insufficient because the resin contained as
much Sn content as 35 ppm.

[0059]In Examples 1-1 and 1-2, the rate of decrease of viscosity was as
small as 5.0%, and spinning ability, service life of the nozzle and
incidence of fluffs during drawing were very excellent, since the content
of Sn in the resin was 50 ppm or less. The tensile strength of the
filament obtained was also excellent showing a level of 4.0 cN/dtex or
more. Particularly, since the rate of decrease of viscosity during
spinning was 7% or less, the degree of polymer degradation during the
spinning process was small with no incidence of break of fibers during
the spinning process, enabling good spinning ability to be obtained as a
result of high tensile strength during the drawing process.

[0060]Tables 1-2 and 1-3 show the changes of spinning ability, service
life of the nozzle and incidence of fluffs during drawing when the
monomer content in the polymer is changed and the results of the quality
of the fiber.

[0061]In Comparative Examples 1-6 to 1-8, the resin was heat-decomposed
during spinning due to particularly large content of the monomer in the
polymer. Spinning was quite difficult due to large decrease of the
viscosity of the polymer during spinning, the service life of the nozzle
was only one day, and a large quantity of fluffs was generated in the
drawing process. The filament obtained had a poor fiber quality with a
tensile strength of less than 3.5 cN/dtex to make the filament to be
practically inapplicable.

[0062]The monomer content was also large in Comparative Example 1-9, and
the resin was inadequate for practical use since the service life of the
nozzle was as short as five days.

[0063]The rate of decrease of viscosity during spinning was improved to 5%
or less in Examples 1-3 to 1-5, since heat decomposition could be
suppressed by reducing the monomer content to 0.5% by weight or less.
Spinning ability, service life of the nozzle and incidence of fluffs
during drawing were also favorable in addition to high tensile strength
of the filament obtained of 4.0 cN/dtex or more.

[0064]Tables 1-4 and 1-5 show the result of spinning with respect to
changes of the pro-portion of L-isomer, presence/absence of the branched
structure, and the molecular weight of the polymer and relative
viscosity.

[0065]Although the polymer in Example 1-6 has similar properties to the
polymer in Comparative Example 1-10 except the presence or absence of the
branched structure, the polymer in Comparative Example 1-10 having the
branched structure has somewhat poor spinning ability while generating
fluffs during drawing, and the tensile strength of the fiber obtained in
the comparative example is lower than 3.5 cN/dtex as compared with that
of the fiber without any branches. Accordingly, the fiber in Comparative
Example 1-10 was practically inapplicable.

[0066]Crystal orientation is not advanced during spinning and drawing in
the fiber in Comparative Example 1-14(Table 1-5) containing less than 95
mol % or less of the L-isomer due to the decreased content of the
L-isomer. The tensile strength thereof was less than 3.5 cN/dtex with a
contraction ratio in boiling water of 30% or more. Therefore, the
filament was practically inapplicable due to poor dimensional stability
in usual wove and knit processing.

[0067]The polymer in Comparative Example 1-11 had so low molecular weight
and relative viscosity that spinning and drawing ability become poor with
a low tensile strength of less than 3.5 cN/dtex. In contrast, the
polymers in Comparative Examples 1-12 and 1-13 had so high molecular
weight and relative viscosity that an elevated spinning temperature was
required. However, the rate of decrease of viscosity during spinning was
increased to 15% by increasing the spinning temperature to deteriorate
spinning and drawing ability with incidence of fluffs during drawing,
thereby making the fiber practically inapplicable.

Multifilament

[0068]The multifilament will be described hereinafter.

[0069]The multifilament can comprises the one constitution element of the
following two constitution elements: [0070](5) a multifilament
comprising a linear polylactic acid containing 98 mol % or more of the
L-isomer, 0 or 30 ppm or less of Sn content and 0 or 0.5% by weight or
less of monomers with a relative viscosity of 2.7 to 3.9; and [0071](6) a
multifilament comprising a linear polylactic acid containing 98 mol % or
more of the L-isomer, 0 or 30 ppm or less of Sn and 0 or 0.5% by weight
or less of monomers with Mw of 120,000 to 220,000 and Mn of 60,000 to
110,000.

[0072]The preferable embodiments of (5) and (6), comprise the following
features: [0073](7) a multifilament having a tensile strength of 3.9
cN/dtex or more, contraction ratio in boiling water of 12% or less,
birefringence (Δn) of 0.025 or more and peak temper-ature of
thermal stress of 85° C. or more; and [0074](8) a multifilament
according to the feature (5) having an inert content of 3.0% or less and
contraction ratio in boiling water of 12% or less. Inert as used herein
means fibers having irregular linear density.

[0075]The process for producing the multifilament comprises the following
two features: [0076](9) a process for producing the polylactic acid
multifilament using the polylactic acid according to the features (5) or
(6) comprising the steps of spinning at a speed of 3,000 m/min or more to
4,500 m/min or less, drawing at a draw magnification factor of 1.3 or
more at a draw temperature of 100 to 125° C., and heat-setting at
125 to 150° C.; and [0077](10) a process for producing the
polylactic acid multifilament using the polylactic acid according to the
features (5) comprising the steps of drawing between the roller heaters
(1) and (2), and heat-setting at the roller heater (2).

[0078]In the conventional method, the polylactic acid biodegradable fiber
is manufactured by spinning at a low speed of 3,000 m/min or less
followed by drawing. Although Japanese Patent Application Laid-open No.
7-216646 and 7-133569 disclose, for example, a producing method in which
a non-drawn polylactic acid fiber spun at a speed of 1000 m/min or less
is reeled and an orientation fiber is obtained in the drawing step,
copolymerization of polyethylene glycol is necessary in the process
disclosed above.

[0079]However, work efficiency of the producing process can be hardly
improved by the processes described above, and it was impossible to
obtain physical and chemical properties and work efficiency comparable to
the fibers made of conventional (non-biodegradable) synthetic resins.

[0080]We strictly surveyed the chemical and physical properties of
polylactic acid as a starting material of the fiber, and have succeeded
in providing a polylactic acid multifilament having such properties as
tensile strength, elongation percentage and contraction ratio in boiling
water comparable to polyester and nylon fibers, as well as being
compatible to post-processing such as weaving, knitting and dyeing as in
the polyester and nylon fibers, by using polylactic acid having selected
properties and by investigating the spinning and drawing steps.

[0081]Polylactic acid to be used herein has a linear structure, or
substantially has no branched structure. It has been proposed in the
former proposal to add a small amount of branching agent in
polymerization of polylactic acid to improve melt viscosity and degree of
polymerization. However, we confirmed that the branched structure of the
resin material far more negatively affects work efficiency of spinning as
compared with conventional polyester fibers in producing the polylactic
acid fiber. Polylactic acid containing even a small amount of the
branched structure exhibits lower tensile strength than polylactic acid
containing no branched structure.

[0082]For excluding the branched structure, it is recommended not to use
any agents such as trivalent or quadrivalent alcohol and carboxylic acids
that arises the branched structure in the polymer material. When the
components having such structure as described above are forced to use for
some reasons, the amount of use should be restricted within a minimum
essential quantity that does not affect work efficiency of spinning such
as break of fibers.

[0083]While polylactic acid comprises L-lactic acid or D-lactic acid, or
L-lactide or D-lactide as a dimer of lactic acid, it is crucial that
lactic acid comprises 98 mol % or more of the L-isomer. This is because
the polymer becomes amorphous when the proportion of the D-isomer
increases and crystal orientation is inhibited in the spinning and
drawing steps, thereby making the properties of the fiber obtained poor.
In particular, the tensile strength is extremely degraded while
excessively increasing the contraction ratio in boiling water to make
practical application of the fiber impossible.

[0084]The polylactic acid contains 0 or 30 ppm or less, preferably 0 or 20
ppm or less, of Sn. While Sn base catalyst used as a polymerization
catalyst of polylactic acid, a residual amount of Sn of over 30 ppm
causes depolymerization during spinning to bring about rapid increase of
the nozzle pressure and extremely decreased work efficiency of spinning.

[0085]To reduce the content of Sn, the amount of Sn used for
polymerization is reduced to be as small as possible, or the chip is
washed with an appropriate solvent.

[0086]The monomer content in the polylactic acid to be used herein is 0.5%
by weight or less, preferably 0.3% by weight or less and in particular 0
or 0.2% by weight or less. The mono-mer refers to the component with a
molecular weight of 1,000 or less as measured by the GPC analysis. Work
efficiency of the fiber decreases due to occurrence of break of fibers in
the spin-ning and drawing steps, when the monomer content exceeds 0.5% by
weight. This is because the monomer component is decomposed by heat to
decrease heat resistance of polylactic acid.

[0087]Unreacted monomers may be removed by evacuating the reaction vessel
just before completing the polymerization reaction, polymerized chips may
be washed with an appropriate liquid, or polylactic acid is synthesized
by solid phase polymerization to reduce the content of monomers in
polylactic acid.

[0088]The polylactic acid preferably has a weight average molecular weight
Mw of 120,000 to 220,000, more preferably 130,000 to 160,000. The
polylactic acid preferably also has a number average molecular weight Mn
of 60,000 to 110,000, more preferably 70,000 to 90,000. While a molecular
weight in this range allows an excellent spinning ability and sufficient
tensile strength to be obtained, a sufficiently high tensile strength
cannot be obtained at a molecular weight as low as out of this range
because large decrease of the molecular weight.

[0089]The polylactic acid has a relative viscosity of 2.7 to 3.9. A
relative viscosity lower than this range makes heat resistance of the
polymer poor, while a relative viscosity higher than this range requires
the spinning temperature to be increased to cause heat degradation during
spinning. The preferable relative viscosity is in the range of 2.9 to
3.3.

[0090]The lower the reduction ratio of the relative viscosity of the
multifilament during spinning is preferable, and the reduction ratio is,
for example, preferably 0 or 7% or less relative to the polymer. The
reduction ratio of 0 or 7% or less substantially causes no decomposition
of the polymer during spinning, makes spinning ability good without
arising break of fibers during spinning, and allows the tensile strength
in the drawing step to be particularly high.

[0091]The multifilament preferably has a tensile strength of 4.0 cN/dtex
or more, because no break of fibers occurs during each processing step. A
birefringence of 0.030 or more is required for increasing the tensile
strength to 4.0 cN/dtex or more.

[0092]The peak temperature of thermal stress of the multifilament is
preferably 85° C. or more, more preferably 90° C. or more,
to prevent dyeing from being fatigued when the multi-filament is dyed
under an atmospheric pressure. A peak temperature of thermal stress of
85° C. or more is preferable since the degree of fatigue of the
dye is reduced.

[0093]The multifilament preferably has an inert content of 3% or less in
the multifilament comprising linear polylactic acid containing 98 mol %
or more of the L-isomer, 0 or 30 ppm or less of Sn and 0 or 0.5% by
weight or less of monomers with a relative viscosity of 2.7 to 3.9. An
inert content of 3% or less is preferable since uneven dyeing seldom
occurs. The more preferable inert content is 1% or less.

[0094]The process for producing the multifilament will be described
hereinafter. The multi-filament is spun at a spinning speed of 3,000
m/min or more and 5,000 m/min or less, drawn at a draw magnification
ratio of 1,3 or more at a draw temperature of 100 to 125° C., and
subjected to heat-setting at 125 to 150° C.

[0095]Crystal orientation becomes insufficient at a spinning speed of less
than 3,000 m/min to make work efficiency of the filament very poor due to
break of fibers at a draw temperature of 110° C. or more. A
spinning speed of exceeding 4,500 m/min makes the filament uneven to
generate uneven spots by cooling, thereby causing unstable work
efficiency of spinning.

[0096]Crystal orientation is prevented from advancing at a draw
temperature of less than 110° C. break of fibers and uneven spots
by drawing causes. Too high draw temperature of exceeding 125° C.
causes break of fibers during the draw step.

[0097]The tensile strength of the fiber becomes as low as less than 4.1
cN/dtex causing many troubles in the processing step such as break of
fibers during weaving and knitting, unless the draw magnification factor
exceeds 1.3. A draw magnification factor of 1.3 or more makes the fibers
available for various processing by adjusting the elongation percentage.
The draw magnification factor is preferably 1.3 to 1.8, more preferably
1.5 to 1.7, considering balance between the tensile strength and
elongation percentage.

[0098]A too low heat-set temperature of lower than 125° C. makes
the contraction ratio in boiling water high, and the fiber cannot be used
due to large contraction in the post-processing. A heat-set temperature
of exceeding 150° C. causes break of fibers since the temperature
is close to the melting point of the polylactic acid fiber. Therefore, a
setting temperature of 135 to 150° C. is preferable considering
productivity of the filament.

[0099]The process for producing the polylactic acid multifilament will be
described hereinafter.

[0100]In the process for producing the polylactic acid multifilament, the
polylactic acid resin having a selected composition and property above
mentioned is melt-spun, drawn between the roller heaters (1) and (2), and
heat-set at the roller heated (2). The producing process is illustrated
in FIG. 1.

[0101]The conventional process is illustrated in FIG. 2. In this process,
the non-drawn fiber 10 is drawn between a roller heater (21) and cold
roller (23), heat-set at a plate heater (22) and rolled up through the
cold roller to obtain rolled drawn fiber 20.

[0102]The roller heater (1) is preferably heated at 100 to 125° C.
for orientation and crystallization of the multifilament in the producing
process.

[0103]The multifilament should be heat-set at the roller heater (2). Using
the roller heater permits the draw point to be fixed at just under the
roller heater (1), thereby enabling the linear density (tex) of the fine
fibers from being uneven.

[0104]The irregular linear density (tex) of the fine fiber is preferably
restricted within ±10%, more preferably within ±7% or less,
relative to the diameter of the multifilament. This range allows
irregular dyeing to be prevented with favorable dyeing.

[0105]The heat-set temperature of the roller heater (2) is preferably in
the range of 125 to 150° C. considering the contraction ratio in
boiling water of the fiber obtained. The temperature is preferably 135 to
150° C. considering productivity of the filament.

Example

[0106]Aspects of this disclosure will be described with reference to
examples.

[0107]The processes for measuring and evaluating each property will be
described first. Measurements and evaluations other than described below
were carried out in accordance with the processes as hitherto described.

Birefringence

[0108]The birefringence of the fiber was measured by a Berek compensator
method using a-bromonaphthaline as an immersion solution.

[0110]A cylindrical knit sample was prepared using the multifilament, and
the sample was dyed under an atmospheric pressure using a disperse dye.
Fatigue of the sample after dyeing was totally evaluated in three steps
of A, B and C: [0111]A: very good (not fatigue at all) [0112]B: good
[0113]C: poor (fatigue is so large that the product is not applicable as
commercial products).Inert--fiber with irregular linear density

[0114]Irregularity in the diameter of the multifilament obtained by a
measuring speed of 50 m/min and twist speed of 5,000 rpm was determined
in percentage using USTER-TESTER 4 made by Zelbeger-Uster Co.

Dyeing

[0115]A test textile was woven using the filament after drawing, and the
textile was dyed under an atmospheric pressure using a disperse dye.
Dyeing of the textile was evaluated in two steps (∘ and x)
based on irregular dyeing, dimensional stability and pilling:
[0116]∘: uniform dyeing [0117]x: irregular dyeing.

Polymerization of Polymer

[0118]Polylactic acid was synthesized by a process known in the art using
L-lactide or D-lactide as a starting material and tin octylate as a
polymerization catalyst. Trimellitic acid in a concentration of 0.1 mol %
as a cross-link agent was added for polymerization for comparison. The
polymer obtained was further polymerized at 135° C. in the solid
phase to reduce the amount of remaining monomers. However, no solid phase
polymerization was applied for a part of the examples as comparative
examples.

Examples 2-1 and 2-2, and Comparative Examples 2-1 to 2-5

[0119]Table 2-1 shows the results of evaluations of spinning ability and
(1), (2) and service life of the nozzle when the polymers with various
contents of Sn are spun at a spinning speed of 3,800 m/min.

[0120]With respect to Comparative Examples 2-1 to 2-3, the polymer was
depolymerized during spinning due to particularly high content of Sn
(residual catalyst). In addition, the rate of decrease of viscosity
during spinning was very high to make spinning quite difficult, and the
service life of the nozzle was as short as 1 day. Therefore, the polymer
in these comparative examples are not practically applicable.

[0121]While the rate of decrease of viscosity during spinning was improved
to 17.6% in the polymer in Comparative Example 2-4, the service life of
the nozzle was only three days due to large content of Sn, which makes
the polymer practically inapplicable.

[0122]The service life of the nozzle was prolonged to six days since the
rate of decrease of viscosity during spinning was improved to 12.3%.
However, the service life of seven days or more could not be attained
since the content of Sn was as high as 35 ppm. The polymers in Examples
2-1 and 2-2 was excellent in spinning ability because the rate of
decrease of viscosity during spinning was as small as 5.0% due to the
small content of Sn of 50 ppm or less with sufficient service life of the
nozzle.

Examples 2-3 to 2-5, and Comparative Examples 2-6 to 2-9

[0123]Table 2-2 shows the results of spinning ability and service life of
the nozzle when the spinning speed was adjusted to 3,500 m/min by varying
the content of the monomer in the polymer.

[0124]With respect to Comparative Examples 2-6 to 2-8, the polymer was
heat-decomposed during spinning due to particularly high content of the
monomer in the polymer. In addition, spinning was quite difficult due to
large rate of decrease of viscosity during spinning besides the service
life of the nozzle was as short as one day, making the polymer
practically inapplicable.

[0125]In the Comparative Example 2-9, the monomer content is still so high
besides the service life of the nozzle is only five days, thereby also
making the polymer practically inapplicable.

[0126]With respect to Examples 2-3 to 2-5, heat decomposition was
suppressed by reducing the monomer content to 0.5% by weight or less.
Consequently, the rate of decrease of viscosity during spinning was
improved to 5% or less, also making spinning ability, service life of the
nozzle and occurrence of fluffs during drawing quite favorable.

[0127]Tables 2-3 and 2-4 show productivity and properties of the
multifilament by changing the proportion of the L-isomer, the molecular
weight and relative viscosity of the polymer with or without the branched
structure with the spinning speed and draw conditions constant, wherein
the contents of Sn and monomers are adjusted to 30 ppm or less and 0.5%
by weight, respectively.

[0128]While the polymers in Example 2-6 and Comparative Example 2-10 have
similar properties with each other except presence/absence of the
branched structure, the polymer having the branched structure in
Comparative Example 2-10 has somewhat poor spinning ability while
generating fluffs during spinning. The tensile strength of the fiber was
less than 3.5 cN/dtex, which is smaller than that of the fiber having no
branched structure, and the peak temperature of thermal stress was
85° C. or less, causing fatigue of dyeing to make the fiber
practically inapplicable.

[0129]Crystal orientation is hardly advanced during spinning and drawing
in the fiber of Comparative Example 2-14 in Table 2-4 having the
proportion of the L-isomer of less than 95 mol %. The tensile strength
thereof is as small as less than 3.5 cN/dtex with the contraction ratio
in boiling water of 30% or more. Therefore, the fiber is practically
inapplicable as the multi-filament due to poor dimensional stability in
usual weave and knit processing.

[0130]Since the fiber of Comparative Example 2-11 has a low molecular
weight and relative viscosity, spinning and drawing ability becomes poor
and the tensile strength thereof is as small as less than 3.5 cN/dtex. In
Comparative Examples 2-12 and 2-13, on the other hand, the molecular
weight and relative viscosity is so high that the spinning temperature is
forced to be elevated. Increasing the spinning temperature results in the
rate of decrease of viscosity during spinning to increase to 15% or more
to make spinning and drawing ability poor with appearance of fluffs
during drawing, thereby making the fiber to be practically inapplicable.

[0131]Tables 2-5 and 2-6 show the results of spinning work efficiency and
properties of the multifilament of the polylactic acid polymer having a
relative viscosity of 3.09, L-isomer content of 98.2 mol % and monomer
content of 0.26% by weight without any branched structure based on the
results in Tables 2-1 to 2-4 when the spinning and drawing conditions are
changed.

[0132]While Example 2-8 and Comparative Example 2-15 show the results
obtained by changing the draw magnification factor of the fibers spun
under the same condition, the fiber with the draw magnification factor of
1.3 or less in Comparative Example 2-15 has so low tensile strength and
birefringence that the multifilament thereof is not suitable for
practical applications.

[0133]Comparative Example 2-16 shows the result obtained by reducing the
spinning speed to 2,800 m/min. However, crystal orientation is so
insufficient at a reel speed of 2800 m/min that the fiber cannot endure
the draw temperature, and break of fiber often occurs to make
productivity of the multifilament low for practical purposes.

[0134]Example 2-9 and Comparative Example 2-17 show the results obtained
by changing the draw temperature after reeling the fibers under the same
condition. Since the draw temperature in Comparative Example 2-17 is
lower than 100° C., break of fibers and generation of fluffs are
often observed due to insufficient draw temperature. The fiber obtained
has so low tensile strength and birefringence that it is not practically
applicable.

[0135]Example 2-9 and Comparative Example 2-18 show the results obtained
by changing the set temperature after reeling the fibers under the same
condition. Since the contraction ratio in boiling water is as high as 20%
or more due to lower set temperature than 125° C. in Comparative
Example 2-18, the fiber is not practically applicable because the
dimensional stability in post-processing such as dyeing is poor.

[0136]Comparative Example 2-19 shows the results obtained by spinning at a
speed exceeding 4,500 m/min. Although vibration of fibers, uneven fibers
by cooling and break of fibers are often observed at a spinning speed of
4,800 m/min to make the fiber practically inapplicable, any problems are
seen with respect to spinning and drawing at the spinning speed of 4,500
m/min in Example 2-10, and the multifilament obtained had good physical
and chemical properties.

[0137]Each polylactic acid polymer was melted at a given temperature and
spun from a nozzle with a nozzle diameter of 0.3 mm. The fiber was reeled
at a speed of 3,000 m/min followed by drawing to prepare a multifilament
with a size of 84 dtex/24f, and dye affinity of the fiber was evaluated.

[0138]Comparative Examples 3-1 and 3-2 show the results when the contents
of residual Sn and monomers are large. Spinning ability is not so good
due to large decrease of viscosity during spinning when the contents of
residual Sn or monomers are large. Generation of fluffs was observed
during drawing and pilling was observed during dyeing, respectively, to
make the quality of the filament poor.

[0139]The quality of the fiber in Comparative Example 3-3 was poor since
the tensile strength was low and generation of fluffs was observed due to
low viscosity and molecular weight (Mw and Mn) of the polymer. The
quality of the fiber in Comparative Example 3-4 was also poor since the
viscosity and molecular weight (Mw and Mn) of the polymer was so high
that the spinning temperature was forced to be elevated, thereby causing
large decrease of viscosity during spinning, and generating fluffs during
drawing and pilling during dyeing.

[0140]While Comparative example 3-5 shows the polymer having similar
properties as the polymer in Example 1 except the presence/absence of the
branched structure, the fiber obtained from the polymer having the
branched structure in Comparative Example 3-5 generated fluffs during
drawing and dye affinity was poor.

[0141]In Comparative Examples 3-7 and 3-8, and in Examples 3-1 and 3-2,
heat-setting after drawing was applied using a roller heater in the
examples and using a plate heater in the comparative examples for the
comparative purposes. The drawing points in the filament are not fixed in
the filament heat-set using the plate heater, inert content and irregular
dying are not improved by changing the set temperature, and the filament
was irregularly dyed to make the filament quality poor. Dye affinity was
good, on the contrary, in the filament prepared by roller heater setting
without arising irregular dying.

[0142]Staple fiber and producing processes thereof will be described in
detail hereinafter.

[0143]Although staple fibers comprising polylactic acid compositions and
producing processes thereof have been disclosed, most of them were in
laboratory levels, and conditions for industrial production have not been
made clear.

[0144]However, assay of the L-isomer in the polylactic acid as a starting
material, prescription of the degree of polymerization of the polymer,
the content of monomers, catalyst and molecular structure as well as rate
of thermal contraction of the staple fibers are crucial factors for
practical production and applications.

[0145]Japanese Patent Application Laid-open No. 6-212511 and 7-11515
disclose briefly spinning and drawing processes of poly-L-lactic acid
with different melt flow rates (MFR), and viscosity characteristics
during melt-spinning of aliphatic polyesters. However, since most of
various conditions required at the practical production site have not
been made clear, it is currently impossible to obtain practically
applicable polylactic acid staple fibers.

[0146]We provide staple fibers of the polylactic acid composition capable
of practical applications with good productivity by using the polylactic
acid composition having selected properties. More particularly, we
provides the staple fibers of the polylactic acid composition having good
thermal contraction characteristics, an excellent tensile strength and
good crimp properties as well as processing stability, and a process for
producing the same.

[0147]Although the polylactic acid composition use L-lactic acid or
D-lactic acid, or L-lactide or D-lactide as a dimer of lactic acid, or
mesolactide as a starting material, it is crucial that the composition
contains 95 mol % or more, preferably 98 mol % or more, of the L-isomer.
Increasing the proportion of the D-isomer makes the polymer amorphous,
and physical and chemical properties of the fiber obtained is
deteriorated due to poor crystal orientation by spinning and drawing. The
tensile strength is particularly decreases and heat contraction ratio
increases to make the fiber to be practically inapplicable.

[0148]The polylactic acid composition has a relative viscosity of 2.7 to
3.9. A sufficient tensile strength cannot be obtained due to poor heat
resistance of the polymer when the relative viscosity is lower than this
range. When the relative viscosity is higher than this range, on the
contrary, the spinning temperature is forced to be elevated to cause
thermal degradation of the polymer during spinning. Accordingly, the
relative viscosity is preferably in the range of 2.9 to 3.6, more
preferably 2.9 to 3.6, because the relative viscosity in this range
permits heat degradation during spinning to be small.

[0149]The lower the rate of decrease of relative viscosity during spinning
is desirable, and the preferable rate is 7% or less. The polymer is
seldom decomposed and break of fibers hardly occurs during spinning when
the rate of decrease of the relative viscosity is less than 7%, thereby
enabling good spinning ability to be attained and the tensile strength in
the drawing step to be large.

[0150]The weight average molecular weight Mw and number average molecular
weight Mn of the polylactic acid composition are preferably in the ranges
of 120,000 to 220,000 and 60,000 to 110,000, respectively. While the
molecular weight in this range affords good spinning ability and
sufficient tensile strength to be attained, the molecular weight out of
this range causes a large decrease in the molecular weight to fail in
obtaining the objective tensile strength.

[0151]The polylactic acid composition has a monomer content of 0.5% by
weight or less, preferably 0.3% by weight or less, and more preferably 0
or 0.2% by weight or less. The monomer refers to the component having a
molecular weight of 1,000 or less as determined by a GPC assay.
Throughput of the process extremely decreases at a monomer content of
more than 0.5% by weight, because heat decomposition of the monomer
decreases heat resistance of the polylactic acid composition.

[0152]For reducing monomer content in the polylactic acid composition,
unreacted monomers are removed by evacuating the reaction vessel at
immediately before completion of the polymerization reaction, polymerized
chips are washed with an appropriate solvent, or the polylactic acid is
manufactured by solid state polymerization.

[0153]The polylactic acid composition is required to contain 30 ppm or
less of Sn, preferably 0 or 20 ppm or less, in the polymer. While an Sn
based catalyst is used as a polymerization catalyst of the polylactic
acid composition, a content of Sn of more than 30 ppm allows spinning
work efficiency to be markedly reduced since the filtration pressure at
the nozzle rapidly increases du to depolymerization during spinning.

[0154]For reducing the content of Sn, the content of Sn for polymerization
is reduced or the chips obtained are washed with an appropriate solvent.

[0155]It is crucial that the polylactic acid composition has a linear
polymer structure, or substantially has no branched structure. Although a
small amount of branching agent was added for improving melt viscosity
and degree of polymerization in polymerizing the polylactic acid
composition in the conventional proposal, we confirmed that the branched
structure of the polylactic acid composition has far more negative effect
on spinning work efficiency than the conventional synthetic fiber, for
example a polyester fiber, has. In other words, the polylactic acid
composition containing even a trace amount of the branched structure has
poor spinning work efficiency and smaller tensile strength as compared
with the composition having no branched structure.

[0156]It is recommended not to use such agents as forming a branched
structure at all in the polymer material, for example three valent or
four valent alcohols and carboxylic acids. When a component having the
structure as described above is forced to use for some reasons, the
quantity thereof should be restricted within as small range as possible
that does not affect spinning work efficiency.

[0157]The polylactic acid preferably exhibits a mass reduction of 5% at a
temperature of 300° C. or more. Thermal degradation in producing
and processing textiles may be more prevented as TG (5%) is higher.

[0158]While commonly used resin components other than polylactic acid may
be used in the polylactic acid staple fiber, biodegradable resin
materials such as aliphatic polyesters are preferably used for the
biodegradable staple fiber.

[0159]The staple fiber of the polylactic acid composition is manufactured
by the steps of melt-spinning the polylactic acid composition by a
conventional method, drawing under a condition to be described
hereinafter, mechanically crimping the spun fiber, and cutting into
staples after heat-treatment.

[0160]The melt-spin temperature is preferably 215 to 250° C.
Melt-extrusion is easy at a temperature of 215° C. or more, and
decomposition may be remarkably suppressed at a temperature of
250° C. or less, thereby enabling high strength staple fibers to
be obtained.

[0161]The fiber after melt-spinning are cooled to ensure a desired crystal
orientation, and are housed in a cans as non-drawn fibers at a speed of
600 to 1200 m/min. A speed less than 600 m/min makes reeling difficult
due to insufficient tension of the fiber, while a speed exceeding 1,200
m/min make it difficult to house in a cans due to high speed spinning.
The speed is preferably 900 to 1,100 m/min.

[0162]The non-drawn fiber is drawn by one or two steps at a draw
temperature of 50 to 98° C. and draw magnification factor of 3.0
to 5.0, preferably 3.5 to 4.5. A draw magnification factor of less than
3.0 is not practical since the elongation is too large, while the
elongation reduces and mechanical load increases and productivity of
drawing reduces when the draw magnification factor exceeds 5.0.

[0163]While the draw magnification factor is different depending on the
spinning speed and required performance of the staple fiber, it is
adjusted so that a fiber having a tensile strength of 2.6 cN/dtex or more
and an elongation of 80% or less is obtained.

[0164]The heat treatment may be applied before or after the crimp
processing. The heat treatment temperature is adjusted to 110 to
150° C., preferably 120 to 140° C., for adjusting the heat
contraction ratio at 120° C. within 5.0%.

[0165]The thermal contraction ratio of the fiber of the polylactic acid
composition staple fiber at 120° C. is preferably 5.0% or less,
more preferably 3.0% or less. The fiber becomes suitable for practical
applications when the thermal contraction ratio at 120° C. is 5.0%
or less, since contraction by heat treatment of the fabric and dyeing
hardly occurs and feeling of the fabric is suppressed from changing when
the staple fiber is processed into a textile product of the spun fiber.
The fiber may be used for the short staple nonwoven fabric through a dry
or wet process, irrespective of thermosetting temperatures.

[0166]The staple fiber of the polylactic acid composition preferably has a
tensile strength of 2.6 cN/dtex or more, more preferably 3.5 cN/dtex or
more. The tensile strength of 2.6 cN/dtex or more is preferable because
no troubles are encountered in the processing step and in practical uses
with a sufficient strength of the final product.

[0167]Practically preferable elongation is 80% or less, more preferably
60% or less.

[0168]The number of crimps of the fiber of the polylactic acid composition
is preferably 4 to 18 crimps/25 mm, more preferably 6 to 15 crimps/25 mm.
Non-dispersed part of the fiber hardly appears when the crimp number more
than 4 crimps/25 mm, while generation of neps is suppressed when the
crimp number is less than 18 crimps/25 mm.

[0169]When the fiber is endowed with crimps by a stuffing box method, tows
before entering the crimper is pre-heated at 40 to 100° C., and
the tows are passed through the crimper with a nip pressure of 0.2 to 0.4
MPa and a press pressure of 0.03 to 0.10 MPa to attain the crimp number
as hitherto described.

[0170]The fiber is heat-treated at 120 to 140° C. for setting the
objective thermal contraction ratio to 5.0% or less.

[0171]Oil may be coated before or after drying, and the fiber is cut with
a cutter to form staple fibers. The staple fiber thus obtained is
excellent in productivity while having good ther-mal contraction
properties, tensile strength and crimp characteristics in addition to
stability in processing.

[0172]The linear density (tex) of a single fiber is usually in the range
of 0.6 to 22 dtex.

[0173]The staple fiber is processed as a woven or knit product by a
conventional weave and knit process, or as a short staple nonwoven fabric
by a dry or wet process.

EXAMPLES

[0174]The disclosure will be described in detail with reference to
examples.

[0175]The analysis processes of the polymer properties and measuring
processes of the textile properties will be described first. The
properties not described hereinafter have been measured and evaluated by
the foregoing processes.

Measurement of Thermal Contraction--Dry Method

[0176]An initial load of 1.8 μN/dtex was given to a sample with a
length of 25 mm to measure the initial length. Then, the length of the
sample after treating with a hot-air dryer at 120° C. for 15
minutes (the sample length after contraction) was measured to determine
the thermal contraction ratio by the equation below:

[0177]Polylactic acid was synthesized by a conventional method using tin
ocrylate as a polymerization catalyst with a starting material ratio of
98.7 mol % of L-lactide and 1.3 mol % of D-lactide. The polymer obtained
had a relative viscosity of 3.02, weight average molecular weight Mw of
146,000 and number average molecular weight Mn of 72,000 with a monomer
content of 0.27% by weight, Sn content of 18 ppm and heat stability
temperature TG (5%) of 318° C.

[0178]The polymer was melt-spun at an extrusion mass rate of 715 g/min and
spinning speed of 1,050 m/min at a spinning temperature of 230° C.
from a spinning nozzle with a diameter of 0.27 mm and number of spinning
holes of 1420. The non-drawn fiber was pulled into a cans after cooling
by in an annular air stream. The rate of decrease of viscosity during
spinning was 3% and the incidence of break of fibers was 0.73 times/ton.

[0179]After pre-heating the non-drawn fiber at 40° C., it was drawn
at a draw magnification factor of 3.96 at 85° C. followed by
heat-treating at 110° C. under a tension. Rill times of on the
roller during drawing was a favorable value of 0.24 times/ton.

[0180]The drawn tows were crimped by introducing into a crimper (a nip
pressure of 0.25 MPa, stuffing pressure of 0.05 MPa) while heating at
85° C. with steam. Then, the crimped tows were dried and
heat-treated at 130° C. with a hot-air dryer. After coating with
an oil, the tows were cut in to a length of 38 mm to obtain staple fibers
with a liner density of 1.1 dtex. The staple fiber obtained had a thermal
contraction ratio at 120° C. of 2.7%, a tensile strength of 4.0
cN/dtex or more, an elongation of 45.4%, and a number of crimps of 10.6
crimps/25 mm. Spinning ability of this staple fiber was good with
satisfactory thermal characteristics and tensile strength of spun fiber.
This staple fiber is mainly used for mix spinning with cotton.

Comparative Example 4-1

[0181]Polylactic acid was synthesized by a conventional method using tin
octylate as a polymerization catalyst with a mixing ratio of the starting
materials of 99.0 mol % of L-lactide and 1.0 mol % of D-lactide together
with 0.1 mol % of trimellitic acid as a cross-link agent.

[0182]The polymer obtained had a relative viscosity of 3.04, a weight
average molecular weight Mw of 148,000, a number average molecular weight
Mn of 76,000, a monomer content of 0.26% by weight and an Sn content of
19 ppm. The heat stability temperature TG (5%) was 315° C.

[0183]A non-drawn fiber was reeled under the same condition as in Example
4-1. Although the rate of decrease of viscosity during spinning was 6%,
spinning ability was not good with an incidence of break of fibers of
2.43 times/ton.

[0184]The non-drawn fiber was drawn under the same condition as in Example
1, whereby rill on the roller during drawing was as poor as 1.21
times/ton.

Example 4-2

[0185]Polylactic acid was synthesized by a conventional method using tin
octylate as a polymerization catalyst with starting material ratios of
97.8 mol % of L-lactide and 2.2 mol % of D-lactide. The polymer obtained
had a relative viscosity of 2.93, weight average molecular weight Mw of
125,000, number average molecular weight Mn of 66,000, monomer content of
0.26% by weight and Sn content of 26 ppm. The heat stability temperature
TG (5%) was 317° C.

[0186]The polymer was melt-spun at a spinning temperature of 230°
C., spinning speed of 950 m/min with an extrusion mass rate of 800 g/min
from a spinning nozzle with a diameter of 0.40 mm and number of spinning
holes of 820. The non-drawn fiber was pulled in cans after cooling in an
annular air stream. The rate of decrease of viscosity during spinning was
5%, and incidence of break of fibers was 0.22 times/ton.

[0187]After preheating the non-drawn fiber at 40° C., the non-drawn
fiber was drawn at a draw magnification factor of 3.74 at 82° C.
Reeling on the roller showed a favorable level of 0.0 times/ton.

[0188]The drawn tows were crimped by introducing into a crimper (nip
pressure of 0.27 MPa and stuffing pressure of 0.06 MPa) while heating
them with steam at 85° C.

[0189]The crimped tows were dried and heat treated at 135° C. with
a hot-air dryer and, after coating with an oil, were cut into a length of
51 mm with a bias length of 76 mm to obtain staple fibers with a linear
density of 3.3 dtex. The staple fiber obtained had a thermal contraction
ratio at 120° C. of 1.7%, tensile strength of 3.0 cN/dtex and
elongation of 58.4% with a number of crimps of 10.9 crimps/25 mm.

[0190]The staple fiber was spun by mixing with wool. The spun fiber had
satisfactory thermal characteristics and tensile strength, and the dyeing
temperature was comparable to poly-esters.

[0191]The staple fibers may be carded to use as a material of a nonwoven
fabric after needle punch and heat treatment.

Example 4-3

[0192]Polylactic acid was synthesized in a starting material composition
of 96.8 mol % of L-lactide and 3.2 mol % of D-lactide by a conventional
method using tin octylate as a polymerization catalyst.

[0193]The polymer obtained had a relative viscosity of 2.96, weight
average molecular weight Mw of 138,000, number average molecular weight
Mn of 80,000, monomer content of 0.47% by weight and Sn content of 19 ppm
with a heat stability temperature TG (5%) of 302° C.

[0194]The polymer was melt-spun at a spinning temperature of 228°
C. and spinning speed of 1,000 in/min with an extrusion mass rate of 800
g/min from a spinning nozzle having 320 holes in the shape of double C
with a slit width of 0.15 mm. The spun fiber was cooled by blowing an
annular air stream, and the non-drawn fiber was pulled in a cans. The
rate of decrease of viscosity during spinning was 5%, and incidence of
break of fibers was 0.0 times/ton.

[0195]After pre-heating the non-drawn fiber at 40° C., it was drawn
at a draw magnification factor of 4.07 at 82° C. Reeling on the
roller during drawing was a favorable level of 0.0 times/ton. The drawn
tow was crimped by introducing into a crimper (nip pressure 0.22 MPa and
stuffing pressure 0.05 MPa) by heating at 85° C. with steam.

[0196]The crimped tow was dried and heat-treated at 130° C. with a
hot-air dryer. After coating with an oil, the tow was cut into a length
of 51 mm to obtain a staple fiber with a linear density of 7.6 dtex.

[0197]The staple fiber obtained had a thermal contraction ratio at
120° C. of 3.5%, tensile strength of 3.4 cN/dtex or more,
elongation of 48.2% and number of crimps of 8.2 crimps/25 mm.

[0198]The staple fiber smoothly passed through the card, and
characteristics of the nonwoven fabric after needle punch and
heat-treatment were satisfactory.

Monofilament and Producing Process Thereof

[0199]The monofilament and producing process thereof will be described
hereinafter.

[0200]Although the monofilament comprising the polylactic acid composition
and producing process thereof have been disclosed, most of them are in a
laboratory level, and conditions for industrial production have not been
made clear.

[0201]However, studies of the composition of polylactic acid as a starting
material, prescription of the degree of polymerization, monomer content,
catalyst and molecular structure as well as thermal contraction
characteristics of the monofilament will be crucial factors for practical
production and applications in the textiles, for particularly
monofilament comprising the polylactic acid composition.

[0202]While Japanese Patent Application Laid-open No. 7-90715 discloses
the polymer viscosity of aliphatic polyesters during spinning and
processes for modifying the polymer, conditions required in the practical
production sites as described above have not been made clear. Therefore,
it has been currently impossible to obtain practically applicable
polylactic acid monofilament.

[0203]We provide a practically applicable monofilament of the polylactic
acid composition with good productivity by using the polylactic acid
composition having selected properties. More particularly, we provide
monofilaments of the polylactic acid composition having good thermal
contraction characteristics and tensile strength capable of stabile
processing, and a process for producing the same.

[0204]While the polylactic acid composition uses L-lactic acid or D-lactic
acid, or L-lactide or D-lactide as a dimer of lactic acid, or mesolactide
as a starting material, it is crucial that the proportion of the L-isomer
is 95 mol % or more, because an increase of the proportion of the
D-isomer brings about an amorphous structure to inhibit crystal
orientation during spinning and drawing from advancing, thereby making
the properties of the textile obtained to be poor. In particular, the
tensile strength is remarkably reduced while increasing thermal
contraction ratio to make the product practically inapplicable.

[0205]The polylactic acid composition to be used in the monofilament has a
relative viscosity (ηrel) of 2.7 to 4.5. Heat resistance of the
polymer becomes poor when the relative viscosity is lower than this range
to fail in obtaining a sufficient tensile strength, while the relative
viscosity of higher than this range forces the spinning temperature to be
elevated to cause heat degradation during spinning.

[0206]The range of the relative viscosity of 2.7 or more and 3.9 or less
is preferable since heat degradation can be suppressed, and more
preferable range is 3.1 to 3.7. However, heat degradation may be
suppressed even when the relative viscosity exceeds 3.9 by adjusting the
content of the L-isomer to 97% or more.

[0207]The lower the rate of decrease of the relative viscosity in spinning
is favorable, and a rate of 7% or less is preferable. When the rate of
decrease of the relative viscosity is less than 7%, the polymer is seldom
decomposed during spinning and break of fibers during spinning hardly
occurs to enable the tensile strength to be large in the draw step with
good spinning ability.

[0208]The polylactic acid composition has a preferable weight average
molecular weight Mw of 120,000 to 220,000, more preferably 150,000 to
200,000, and a preferable number average molecular weight Mn of 60,000 to
110,000, more preferably 80,000 to 100,000. While a molecular weight
within this range permits good spinning ability and sufficient tensile
strength to be obtained, a large decrease of the molecular weight causes
to make it impossible to obtain a required tensile strength when the
molecular weight is out of this range.

[0209]The polylactic acid composition has a monomer content of 0.5% by
weight or less, preferably 0.3% by weight or less and more preferably 0
or 0.2% by weight or less. The monomer is referred to as the monomer
component having a molecular weight of 1,000 or less as determined by a
GPC assay. The monomer content of exceeding 0.5% by weight markedly
decreases work efficiency of the polymer, because the monomer component
is decomposed by heat to decrease heat resistance of the polylactic acid
composition.

[0210]For reducing the content of the monomer in the polylactic acid
composition, the unreacted monomers are removed by evacuating the
reaction vessel at immediately before completion of the polymerization
reaction, the polymerized chips are washed with an appropriate solvent,
or the polylactic acid is polymerized by solid state polymerization.

[0211]It is essential that the polylactic acid composition contains 30 ppm
or less, preferably 0 or 20 ppm or less, of Sn in the polymer. While the
Sn based catalyst is used as the polymerization catalyst of the
polylactic acid composition, a content of Sn of exceeding 30 ppm allows
the polymer to be depolymerized during spinning to rapidly increase
filtration pressure of the spinning nozzle, thereby remarkably reducing
work efficiency of spinning.

[0212]For reducing the content of Sn the amount of Sn for polymerization
may be reduced, or the polymer may be washed with an appropriate solvent.

[0213]It is essential that the polylactic acid composition has a linear
polymer structure, or substantially contains no branched structure. A
small amount of branching agent have been added for polymerization of the
polylactic acid composition for the purpose of improving the melt
viscosity and degree of polymerization. However, we confirmed that the
branched structure of the polylactic acid composition far more negatively
affects spinning work efficiency as compared with conventional
monofilaments, for example polyester monofilaments. In other words, the
polylactic acid composition containing even a small amount of the
branched structure is poor in spinning work efficiency besides having a
lower tensile strength than the structure without any branched structure.

[0214]For excluding the branched structure, it is recommended to avoid use
of agents that arise the branched structure, for example three valent or
four valent alcohols and carboxylic acids, in the polymer material.
However, when a component having such structure is forced to use for some
reasons, the amount should be restricted within a minimum essential range
that does not affect work efficiency of spinning.

[0215]The polylactic acid preferably has a mass reduction of 5% at a
temperature of 300° C. or more, or has a heat stability
temperature of TG (5%) of 300° C. or more. Thermal degradation in
producing and processing textiles may be more prevented as TG is higher.

[0216]Although common resins other than polylactic acid may be used as
starting materials in the polylactic acid monofilament, the material is
preferably a biodegradable resin such as an aliphatic polyester for
manufacturing a biodegradable monofilament.

[0217]The monofilament of the polylactic acid composition is manufactured
by melt-spinning the polymer by a conventional method at 220 to
250° C. followed by cooling with water, and heat-treating after
heat-drawing under the following conditions.

[0218]The melt-spinning temperature is preferably 220 to 250° C.,
because melt-extrusion becomes easy at a temperature of 220° C. or
more, and decomposition is extremely suppressed at a temperature of
250° C. or less, thereby enabling a monofilament having a high
tensile strength to be easily obtained.

[0219]The melt-spun filament is drawn at a prescribed temperature and draw
magnification factor while cooling with water to facilitate a given
crystal orientation, and the filament is reeled on a bobbin. The
non-drawn filament is drawn by one or two steps or more in hot water at
70 to 100° C., preferably at 85 to 98° C.

[0220]The draw magnification factor is 6.0 or more, preferably 8.0 or
more. The factor is different depending on the required performance of
the filament, and is determined so that a filament having a tensile
strength of 3.5 cN/dtex or more and elongation of 40.0% or less is
obtained. The heat-treatment temperature is adjusted in the range of 100
to 150° C., preferably 120 to 140° C., for restricting the
contraction ratio in boiling water to 10.0% or less.

[0221]The contraction ratio in boiling water of the monofilament of the
polylactic acid composition is preferably 10.0% or less, more preferably
8.0% or less.

[0222]The filament is favorable for practical uses since the filament is
hardly contracted by heat-treatment without causing any changes in the
feeling when the contraction ratio in boiling water is 10.0% or less.
There will be also no problem of making the use of the textile impossible
depending on the heat-setting temperature.

[0223]The monofilament of the polylactic acid composition preferably has a
tensile strength of 3.5 cN/dtex or more, more preferably 4.4 cN/dtex or
more.

[0224]No troubles will be encountered in the processing steps when the
tensile strength is 3.5 cN/dtex or more with a sufficient strength of the
final product to exclude troubles in practical applications.

[0225]The elongation is preferably 40.0% or less, more preferably 35.0% or
less, from the practical point of view.

[0226]The birefringence Δn after drawing is preferably 0.0250 or
more, more preferably 0.0330 or more. Crystal orientation sufficiently
advances and contraction ratio in boiling water is properly suppressed
when the filament has a birefringence Δn of 0.0250 or more.

[0227]The monofilament obtained as described above is excellent in
productivity while having practically applicable thermal contraction
ratio and tensile strength as well as stability in processing.

[0228]The monofilament usually has a linear density of 220 to 1,100 dtex.

[0229]The monofilament can be used as woven and knit fabrics manufactured
by the process known in the art.

Example 5-1

[0230]Polylactic acid was synthesized by the conventional method using tin
octylate as a polymerization catalyst with a starting material ratio of
96.0 mol % of L-lactide and 4.0 mol % of D-lactide.

[0231]The polymer obtained had a relative viscosity of 3.7, weight average
molecular weight Mw of 195,000, number average molecular weight Mn of
94,000, monomer content of, 0.27% or less by weight and Sn content of 17
ppm with a heat stability temperature (5%) of 319° C.

[0232]The polymer was melted at 220° C. in a single screw extruder,
and was extruded from a nozzle having 18 spinning holes with a diameter
of 1.2 mm. After allowing the filament to pass through a cooling water
bath, it was subjected to a first step drawing at a draw magnification
factor of 5.5 in hot water at 94° C., and to a second step drawing
at a draw magnification factor of 1.2 in hot water at 98° C.,
followed by heat-setting in a hot air stream at 130° C. to
manufacture a monofilament with a linear density of 560 dtex.

[0233]The monofilament obtained had a contraction ratio in boiling water
of 9.3%, tensile strength of 4.4 cN/dtex, elongation of 36%, and
birefringence tin of 0.0325. The rate of decrease of viscosity during
spinning was 4%, suggesting small amount of decomposition of the polymer
during spinning to result in substantially no break of fibers.

[0234]The contraction ratio in boiling water of 10.0% or less allows the
woven and knit fabric to hardly contract by heat-treatment without any
changes in the feeling, thus making the product to be practically
applicable. No troubles of making the fabric unusable by the heat-setting
temperature was encountered. The tensile strength of 3.5 cN/dtex or more
prevents troubles in the processing steps from occurring, and allows the
strength of the final product to be sufficient without generating
practical problems. The elongation of 40.0% or less is suitable for
practical applications. The birefringence of 0.0320 or more indicate well
advanced crystal orientation and adequately suppressed contraction ratio
in boiling water.

Comparative Example 5-1

[0235]Polylactic acid was synthesized by the conventional method using
L-lactide and D-lactide as starting materials and tin octylate as a
polymerization catalyst, and by adding 0.1 mol % of trimellitic acid as a
cross-linking agent.

[0236]The polymer obtained contained 95.5 mol % of the L-isomer and had a
relative viscosity of 3.7, weight average molecular weight Mw of 185,000,
number average molecular weight Mn of 92,000, monomer content of 0.8% by
weight and Sn content of 16 ppm with a thermal stability temperature (5%)
of 320° C.

[0237]The polymer was melted at 220° C. in a single screw extruder
and extruded from a nozzle having 18 spinning holes with a diameter of
1.2 mm.

[0238]The filament was passed through a water cooling bath, subjected to a
first step drawing with a draw magnification factor of 5.5 in hot water
at 94° C. and second step drawing with a draw magnification factor
of 1.2 in hot water at 98° C., and heat set at 130° C. in a
hot air stream to manufacture a monofilament with a linear density of 560
dtex. However, this filament was poor in spinning ability with high
incidence of break of fibers due to large proportion of cross-linked
polylactic acid.

Example 5-2

[0239]Polylactic acid was synthesized by a conventional method with a
starting material ratio of 95.7 mol % of L-lactide and 4.3 mol % of
D-lactide using tin octylate as a polymerization catalyst.

[0240]The polymer obtained had a relative viscosity of 3.3, weight average
molecular weight Mw of 174,000, number average molecular weight Mn of
91,000, monomer content of 0.20% by weight or less and Sn content of 16
ppm with a heat stability temperature (5%) of 319° C.

[0241]The polymer was melted at 220° C. in a single screw extruder,
and extruded from a nozzle having 18 spinning holes with a diameter of
1.2 mm. The filament was passed through a water cooling bath, and
subjected to the first step drawing at a draw magnification factor of 6.0
in hot water at 94° C. and second step drawing at a draw
magnification factor of 1.5 in hot water at 98° C. The drawn
filament was heat-set at 130° C. in a hot air stream to
manufacture a mono-filament with a linear density of 560 dtex.

[0242]The monofilament obtained had a contraction ratio in boiling water
of 6.7%, tensile strength of 5.1 cN/dtex, elongation of 33.0% and
birefringence Δn of 0.0350. The rate of decrease of viscosity
during spinning of 4% suggests a small amount decomposition of the
polymer during spinning with substantially no break of fibers.

[0243]The contraction ratio in boiling water of 10.0% or less affords
practically favorable woven and knit products due to seldom contraction
during heat-treatment with no changes in feeling. Troubles such that the
product becomes unusable by heat-setting temperature could be also
avoided.

[0244]The tensile strength of 3.5 cN/dtex or more hardly arises troubles
in the processing steps with sufficient strength in the final products
avoiding any troubles in practical applications. The elongation of 40.0%
or less was practically favorable.

[0245]The birefringence of 0.0320 or more indicates sufficiently advanced
crystal orientation to adequately suppress the contraction ratio in
boiling water.

Example 5-3

[0246]Polylactic acid was synthesized by the conventional method using tin
octylate as a polymerization catalyst with a starting material ratio of
98.9 mol % of L-lactide and 1.1 mol % of D-lactide.

[0247]The polymer obtained had a relative viscosity of 4.5, weight or less
average molecular weight of 230,000, number average molecular weight of
116,000, monomer content of 0.2% by weight or less and Sn content of 16
ppm with a heat stability temperature (5%) of 319° C.

[0248]The polymer was melted at 228° C. in a single screw extruder,
and extruded from a nozzle having 18 spinning holes with a diameter of
1.2 mm. The filament was passed through a water cooling bath, and
subjected to the first step drawing with a draw magnification factor of
6.0 in hot water at 98° C. and the second step drawing with a draw
magnification factor of 1.85 in hot water at 98° C. with a total
draw magnification factor of 11.1. The filament was heat-set in a hot air
stream at 130° C. to manufacture a monofilament with a linear
density of 560 dtex.

[0249]The monofilament obtained had a contraction ratio in boiling water
of 4.2%, contraction ratio after hot air treatment at 100° C. of
3.1%, tensile strength of 5.15 cN/dtex and elongation of 28.0%. The rate
of decrease of viscosity during spinning of 4% suggests small amount of
decomposition of the polymer during spinning to substantially arise no
break of fibers.

[0250]The contraction ratio in boiling water of 6.0% or less and
contraction ratio after hot air treatment at 100° C. of 4.0%
afford woven and knit products that scarcely arise contraction during
heat-treatment. The product substantially shows no changes of feeling
that makes the product practically favorable.

[0251]The tensile strength of 4.85 cN/dtex or more can prevent troubles in
the processing steps with sufficient strength of the final product
without any practical problems. The elongation of 30.0% or less was
practically favorable.

Flat Yarn and Producing Process Thereof

[0252]The flat yarn and producing process thereof will be described
hereinafter.

[0253]In textile products from the polylactic acid composition, in
particular the flat yarn among them, the composition of polylactic acid
as a starting material, prescription of the degree of polymerization of
the polymer, the monomer content, catalyst and molecular structure as
well as thermal contraction characteristics of the flat yarn are crucial
factors for practical producing and uses.

[0254]For example, Japanese Patent No. 2733184 discloses a flat yarn
manufactured by melt extrusion molding of an aliphatic polyester
comprising glycolic acid and polybasic acid as constituents. However,
only the prior art is described with respect to lactic acid, and no
detailed explanation is made in the patent. Conditions required at
practical production sites have not been made clear. Therefore, it is
currently impossible to obtain practically applicable polylactic acid
flat yarns.

[0255]We thus provide a practically applicable polylactic acid flat yarn
with high productivity by using a polylactic acid composition having
selected properties. More particularly, we provide a polylactic acid flat
yarn having good thermal contraction characteristics and high tensile
strength as well as stability in processing and producing process
thereof.

[0256]While the starting material of the polylactic acid composition
comprises L-lactic acid or D-lactic acid, or L-lactide or D-lactide as a
dimer of lactic acid, or mesolactide, it is crucial that the proportion
of the L-isomer is 95 mol % or more. This is because increased proportion
of the D-isomer results in an amorphous structure, which prevent crystal
orientation by drawing from advancing to make the properties of the
textile obtained poor. The tensile strength particularly decreases while
increasing the thermal contraction ratio to make practical applications
of the textile impossible.

[0257]The polylactic acid composition has a relative viscosity (vet) of
2.7 to 4.5. The melt-extrusion temperature should be elevated when the
viscosity exceeds the upper limit to consequently increase thermal
degradation while, when the viscosity is below the lower limit, heat
resistance of the polymer becomes too poor to obtain a sufficient tensile
strength. Accordingly, the preferable range of the relative viscosity is
3.3 to 4.3.

[0258]The lower the rate of decrease of viscosity during melt extrusion is
favorable, and preferable rate is 7% or less. The polymer is not
substantially decomposed by melt-extrusion when the rate of decrease of
viscosity during melt extrusion is 7% or less to exclude irregular films
from being formed by melt-extrusion. Accordingly, a film having a high
tensile strength during drawing may be formed with good film forming
ability.

[0259]The polylactic acid composition preferably has a weight average
molecular weight Mw of 125,000 to 230,000, more preferably 174,000 to
215,000, and number average molecular weight Mn of 73,000 to 116,000,
more preferably 91,000 to 107,000. The molecular weight in this range
permits good film forming ability and high tensile strength to be
obtained.

[0260]The polylactic acid composition contains 0.5% by weight or less,
preferably 0.3% by weight or less, and more preferably 0 or 0.2% by
weight or less of monomers. The monomer refers to as a monomer component
having a molecular weight of 1000 or less as determined by a GPC assay.
The monomer content of 0.5% by weight or less is preferable for attaining
high work efficiency, because heat resistance of the polylactic acid
composition becomes more excellent as the content of the heat-decomposed
monomer component is smaller.

[0261]For reducing the monomer content in the polylactic acid composition,
unreacted monomers may be removed by evacuating the reaction vessel
immediately before completing the polymerization reaction, polymerized
chips may be washed with an appropriate solvent, or polylactic acid is
prepared by solid phase polymerization.

[0262]The content of Sn in the polylactic acid composition is required to
be 30 ppm or less, preferably 0 or 20 ppm or less. While the Sn based
catalyst is used as a polymerization catalyst of the polylactic acid
composition, a content of 30 ppm or less permits filtration pressure at
the nozzle to be suppressed from increasing due to small amount of
depolymerization during melt-extrusion to make the polymer excellent in
melt-extrusion ability.

[0263]For reducing the content of Sn, the proportion of Sn used in
polymerization is reduced, or the chips are washed with an appropriate
solvent.

[0264]It is essential that the polylactic acid composition has a linear
polymer structure, or substantially has no branched structure. It has
been proposed to add a small amount of a branching agent for preparing
the polylactic acid composition to improve the melt viscosity and degree
of polymerization. However, we confirmed that the branched structure of
the polylactic acid composition far more negatively affects film forming
ability as compared with conventional flat yarns, for example polyester
flat yarns. In other words, it is a problem that work efficiency for
forming the film becomes poor in the polylactic acid composition
containing even a small quantity of branched structures, and tensile
strength of the film is lower as compared with the film having no
branched structures.

[0265]For excluding the branched structure, it is recommended to avoid use
of agents that arise the branched structure, for example three valent or
four valent alcohols and carboxylic acids, in the polymer material.
However, when a component having such structure is forced to use for some
reasons, the amount should be restricted within a minimum essential range
that does not affect the film forming ability.

[0266]The polylactic acid preferably has a temperature for reducing the
polymer mass by 5%, or TG (5%), of 300° C. or more. The higher TG
(5%) is, the more heat degradation in producing and processing the flat
yarn may be prevented.

[0267]Although common resins other than polylactic acid may be used as
starting materials in the polylactic acid flat yarn, the material is
preferably a biodegradable resin such as an aliphatic polyester for
manufacturing a biodegradable flat yarn.

[0268]While the flat yarn of the polylactic acid composition may be
manufactured by a process known in the art using the polymer of the
polylactic acid composition, one example of the producing process
comprises solidifying by cooling after melt-extrusion, and hot-drawing
under the conditions to be described below followed by heat-treatment.

[0269]The melt-extrusion temperature is preferably in the range of 180 to
250° C. A melt-extrusion temperature of 180° C. or more
makes melt-extrusion easy, while a temperature of 250° C. or less
extremely prevent decomposition, thereby enabling a flat yarn having a
high tensile strength to be easily obtained.

[0270]The melt-extruded film is cooled to attain a desired crystal
orientation, and drawn at a prescribed temperature and draw magnification
factor followed by reeling on a bobbin after heat-treatment. The film is
slit into ribbons, which are drawn by one or two steps at 80 to
130° C., preferably at 100 to 120° C.

[0271]The draw magnification factor is 4.0 or more, preferably 5.0 or
more. Although the factor differs depending on the required performance
of the objective flat yarn, it is determined so that a flat yarn having a
tensile strength of 2.6 cN/dtex or more and elongation of 40.0% or less
is obtained.

[0272]The flat yarn is preferably heat-treated at 100 to 150° C.,
more preferably at 110 to 140° C., for adjusting the contraction
ratio after heat-treatment at 80° C. for 10 minutes to 5.0% or
less.

[0273]The flat yarn of the polylactic acid composition preferably has
preferably a contraction ratio of 5.0% or less, more preferably 3.0% or
less, after heat-treating the flat yarn at 80° C. for 10 minutes.
The contraction ratio of 5.0% or less after heat-treating the flat yarn
at 80° C. for 10 minutes allows contraction by heat-treatment to
be hardly occurs when the yarn is processed into woven and knit fabrics
without any changes of feeling. Therefore, the fabric is favorable for
use by excluding the problems that the fabric becomes unusable by
heat-setting temperature.

[0274]The flat yarn of the polylactic acid composition preferably has a
tensile strength of 2.6 cN/dtex or more, more preferably a tensile
strength of 3.0 cN/dtex or more. A tensile strength of 2.6 cN/dtex or
more seldom arises troubles in the processing steps besides having a
sufficient strength in the final product by excluding practical problems.

[0275]The elongation is preferably 40.0% or less, more preferably 35.0% or
less, from the practical point of view.

[0276]The flat yarn thus obtained is excellent in productivity, and has
good thermal contraction characteristics and tensile strength suitable
for practical uses as well as stability in processing.

[0277]The linear density of the flat yarn is usually in the range of 330
to 1100 dtex when the yarn has a width of 3 to 6 mm, and 560 to 3,300
dtex when the yarn has a width of 6 to 12 mm.

[0278]The flat yarn may be processed in to woven and knit fabrics for use
by the process known in the art.

EXAMPLES

[0279]Additional examples will be described hereinafter with reference to
measurements of physical and chemical properties. The properties not
described below was measured by the process as hitherto described.

Rate of Decrease of Viscosity During Melt-Extrusion

[0280]The relative viscosity (ηrel) of the film shaped sample extruded
out of the die was measured to determine the rate of decrease of
viscosity by the following equation. The residence time of the molten
polymer was about 10 minutes in this example:

Rate of decrease of viscosity during melt-extrusion(%)=((relative
viscosity of polymer-relative viscosity of film)/relative viscosity of
polymer)×100.

Example 6-1

[0281]Polylactic acid was synthesized by a conventional process using tin
octylate as a polymerization catalyst with a starting material ratio of
96.0 mol % of L-lactide and 4.0 mol % of D-lactide.

[0282]The polymer obtained had a relative viscosity of 3.7, weight average
molecular weight Mw of 195,000, number average molecular weight Mn of
94,000, monomer content of 0.27% by weight or less and Sn content of 17
ppm with a heat stability temperature (5%) of 319° C.

[0283]The polymer was melted in a single screw extruder at 190° C.,
and melt-extruded from a circular die molding apparatus with a diameter
of 30 cm and a lip gap of 1.0 mm, followed by solidifying by cooling to
form a raw sheet. The raw sheet was slit into 6 mm wide strips, which
were drawn on a hot plate followed by anneal drawing with a hot air
stream. The first step drawing was performed on a hot plate at a
temperature of 115° C. with a draw magnification factor of 5.0,
and the second step drawing was performed on a hot plate at a temperature
of 120° C. with a draw magnification factor of 1.2, followed by
heat-setting at 130° C. in a hot air stream with an annealing
ratio of 5%, thereby obtaining a flat yarn with an width of 3 mm and
linear density of 560 dtex.

[0284]The flat yarn obtained had a contraction ratio of 3.9%, tensile
strength of 2.9 cN/dtex and elongation of 33.0%. The rate of decrease of
viscosity during melt-extrusion of 4% suggests small amount of
decomposition of the polymer during melt-extrusion to substantially arise
no troubles in forming the raw sheet. The contraction ratio of 5.0% or
less allows contraction by heat-treatment to be hardly generated when the
flat yarn is processed into woven and knit fabrics with no changes in
feeling, making the fabrics practically favorable. Problems that the
textile becomes unusable by the heat-setting temperature were never
observed. Since the tensile strength is 2.6 cN/dtex or more, no troubles
were encountered in the processing steps to ensure sufficient strength of
the final product to exclude practical problems. The elongation of 40.0%
or less was practically favorable.

Comparative Example 6-1

[0285]Polylactic acid was synthesized by the conventional method using tin
octylate as a polymerization catalyst and L-lactide and D-lactide as
starting materials, and by adding 0.1 mol % of trimellitic acid as a
cross-linking agent. The polymer obtained contained the 95.5 mol % of
L-isomer and had a relative viscosity of 3.7, weight average molecular
weight Mw of 185,000, number average molecular weight Mn of 92,000,
monomer content of 0.18% by weight or less and Sn content of 16 ppm with
a heat stability temperature (5%) of 320° C.

[0286]The polymer was melted in a single screw extruder at 190° C.,
and melt-extruded from a circular die extruder with a diameter of 30 cm
having a lip gap of 1.0 mm, followed by solidifying by cooling to form a
raw sheet. Since the sheet contains cross-linked polylactic acid, many
troubles were seen in forming the raw sheet with poor melt-extrusion
ability. The raw sheet was slit into 6 mm wide stripes, which were drawn
on a hot plate followed by anneal drawing with a hot air stream. The
first step drawing was performed on a hot plate at a temperature of
118° C. with a draw magnification factor of 5.0, and the second
step drawing was per-formed on a hot plate at a temperature of
120° C. with a draw magnification factor of 1.2, followed by
heat-setting at 125° C. in a hot air stream with an annealing
ratio of 5%, thereby obtaining a flat yarn with an width of 3 mm and
linear density of 560 dtex. Troubles during drawing the flat yarn was
often seen due to the presence of cross-linked polylactic acid in the
polymer in addition to poor drawing ability.

Examples 6-2

[0287]Polylactic acid was synthesized by the conventional method using tin
octylate as a polymerizing catalyst with a starting material ratio of
95.7 mol % of L-lactide and 4.3 mol % of D-lactide.

[0288]The polymer obtained had a relative viscosity of 3.3, weight average
molecular weight Mw of 174,000, number average molecular weight Mn of
91,000, monomer content of 0.20% by weight or less, and Sn content of 16
ppm with a heat stability temperature (5%) of 319° C.

[0289]The polymer was melted in a single screw extruder at 190° C.,
and melt-extruded from a circular die extruder having a diameter of 30 cm
with a lip gap of 1.0 mm, followed by solidification by cooling to form a
raw sheet. This sheet was slit into 6 mm wide stripes, which were drawn
on a hot plate followed by annealing heat-treatment in a hot air stream.
The first step drawing was performed on a hot plate at a temperature of
115° C. with a draw magnification factor of 5.5, and the second
step drawing was performed on a hot plate at a temperature of 120°
C. with a draw magnification factor of 1.2, followed by heat-setting at
130° C. in a hot air stream with an annealing ratio of 5%, thereby
obtaining a flat yarn with an width of 3 mm and linear density of 890
dtex.

[0290]The flat yarn obtained had a contraction ratio of 4.3%, tensile
strength of 2.7 cN/dtex and elongation of 36.0%. The rate of decrease of
viscosity during melt-extrusion of 4% suggests a small amount of
decomposition of the polymer to avoid troubles in forming the raw sheet.
The contraction ratio of 5.0% or less hardly generates contraction by
heat-treatment when the yarn is processed into woven and knit fabrics
with no changes of feeling, which is suitable for practical application.
Problems that the fabric becomes unusable by the heat-setting temperature
were also avoided. The tensile strength of 2.6 cN/dtex or more hardly
arises troubles in the processing steps to make the strength of the final
product sufficient without any practical problems. The elongation of
40.0% or less was practically favorable.

Example 6-3

[0291]Polylactic aid was synthesized by the conventional method using tin
octylate as a polymerizing catalyst with a starting material ratio of
98.5 mol % of L-lactide and 1.5 mol % of D-lactide.

[0292]The polymer obtained had a relative viscosity of 4.2, weight average
molecular weight Mw of 201,000, number average molecular weight Mn of
103,000, monomer content of 0.20% by weight or less and Sn content of 16
ppm with a heat stability temperature (5%) of 319° C.

[0293]The polymer was melted in a single screw extruder at 190° C.,
and melt-extruded from a circular die extruder having a diameter of 30 cm
with a lip gap of 1.0 mm, followed by solidification by cooling to form a
raw sheet. This sheet was slit into 6 mm wide stripes, which were drawn
on a hot plate followed by annealing heat-treatment in a hot air stream.
The first step drawing was performed on a hot plate at a temperature of
118° C. with a draw magnification factor of 5.5, and the second
step drawing was performed on a hot plate at a temperature of 120°
C. with a draw magnification factor of 1.2, followed by heat-setting at
130° C. in a hot air stream with an annealing ratio of 5%, thereby
obtaining a flat yarn with an width of 3 mm and linear density of 890
dtex.

[0294]The flat yarn obtained had a contraction ratio of 1.9%, tensile
strength of 3.4 cN/dtex and elongation of 30.0%. The rate of decrease of
viscosity during melt-extrusion of 4% suggests a small amount of
decomposition of the polymer to avoid troubles in forming the raw sheet.

[0295]The contraction ratio of 5.0% or less hardly generates contraction
by heat-treatment when the yarn is processed into woven and knit fabrics
with no changes of feeling, which is suitable for practical application.
Problems that the fabric becomes unusable by the heat-setting temperature
were also avoided. The tensile strength of 2.6 cN/dtex or more hardly
arises troubles in the processing steps to make the strength of the final
product sufficient without any practical problems. The elongation of
40.0% or less was practically favorable.

False-Twist Yarn and Producing Process Thereof

[0296]The false-twist yarn and producing process thereof will be described
hereinafter.

[0297]A long term operation is difficult in the false-twist yarn
manufactured from a biodegradable resin currently known in the art
because break of yarns during processing frequently happens. Moreover,
the tensile strength and expansion-contraction recovery ratio are so low
that crimp characteristics required for the false-twist yarn is extremely
poor. It is also a problem that a high quality fabric cannot be
constantly supplied due to frequently occurring break of yarns and fluffs
in the post processing such as weave and knit processing.

[0298]We invented false-twist yarns excellent in work efficiency and
properties by using polylactic acid having selected properties through
intensive studies of the properties of polylactic acid as a starting
material of the false-twist yarn. It could also be helpful to provide a
practically applicable false-twist yarn comprising polylactic acid with
excellent work efficiency, wherein the polylactic acid fiber is capable
of processing into a twist yarn, wherein the polylactic acid twist yarn
is free from break of yarns and filament with excellent characteristics
as textiles, and wherein the twist yarn has physical properties such as
tensile strength and expansion/contraction recovery ratio comparative to
those of conventional polyester twist yarns, and is to provide the
processes for producing thereof.

[0299]The false-twist yarn satisfies the following features: [0300]In a
first aspect, we provide a false-twist yarn mainly comprising a
polylactic acid resin, wherein the monomer content in the polylactic acid
is 0.5% by weight or less. [0301]In a second aspect according to the more
preferred embodiment of the first aspect, the polylactic acid false-twist
yarn comprises 95 mol % or more of the L-isomer of the polylactic acid
resin. [0302]In a more preferable third aspect, the polylactic acid
false-twist yarn according to the first and second aspects comprises a
linear polylactic acid resin. [0303]In a further preferable fourth
aspect, the polylactic acid false-twist yarn according to the first to
third aspects comprises the polylactic acid resin with ηrel of 2.7 to
3.9. [0304]In a more preferable fifth aspect, the polylactic acid
false-twist yarn according to the first to fourth aspect comprises the
polylactic acid resin with an Sn content of 0 or 30 ppm or less. [0305]In
a more preferable sixth aspect, the polylactic acid false-twist yarn
according to the first to fifth aspects has a tensile strength of 2.4
cN/dtex or more. [0306]In a more preferable seventh aspect, the
polylactic acid false twist yarn according to the first to sixth aspects
has a expansion/contraction recovery ratio of 10% or more.

[0307]In the process for producing the polylactic acid false-twist yarn as
described above, a polylactic acid non-drawn yarn is subjected to a
simultaneous draw and false-twist processing at a draw temperature of
110° C. or more and draw magnification factor of 1.3 to 1.8,
wherein the polylactic acid resin according to the first to fifth aspects
has birefringence Δn of 0.010 to 0.035, the tensile strength S
(cN/dtex) and ultimate elongation E (%) is represented by the relation of
15≦S× E≦23.

[0308]The monomer content in polylactic acid is required to be 0 or 0.5%
by weight or less. Monomers refer to the component having a molecular
weight of 1,000 or less as determined by a GPC assay. Yarns are liable to
be fragile and the twisted yarn suffers extreme stress when the monomer
content exceeds 0.5% by weight, thereby the tensile strength is markedly
decreases. Throughput of twist works turn out to be unstable due to
frequent break of yarns during the process by the same reason as
described above.

[0309]Usually, the reaction vessel is evacuated immediately before
completing the polymerization reaction for removing unreacted monomers in
the polylactic acid. Otherwise, polymerized chips may be washed with an
appropriate solvent, or subjected to a solid state polymerization.

[0310]Lactic acid comprises naturally occurring L-lactic acid and D-lactic
acid as an optical isomer of L-lactic acid, L-lactide and D-lactide as
dimers thereof, and mesolactide. The proportion of L-isomer is preferably
95 mol % or more, more preferably 98 mol % or more.

[0311]When the proportion of the L-isomer is 95 mol % or more, the resin
becomes highly heat resistant to allow the tensile strength of the yarn
to be seldom decreased even by heat-setting at a relatively high
temperature. Heat-setting at a high temperature makes
expansion/contraction recovery ratio of the yarn to be excellent to
enable a false-twist yarn with excellent crimp characteristics to be
obtained.

[0312]The polylactic acid is preferably a linear polymer, or substantially
has no branched structure. Adding a branching agent in the polymerization
process of polylactic acid has been proposed for improving melt viscosity
and degree of polymerization. However, we confirmed that the branched
structure of the polylactic acid composition far more negatively affects
properties of the false-twist yarn and work efficiency of the yarn as
compared with conventional polyesters. In other words, the multifilament
comprising polylactic acid having no branched structure seldom arises
break of yarns during false-twisting, and the false-twist yarn obtained
therefrom has a higher tensile strength than the false-twist yarn having
some branched structure.

[0313]For excluding the branched structure, it is recommended to avoid use
of agents that arise the branched structure, for example three valent or
four valent alcohols and carboxylic acids, in the polymer material.
However, when these chemicals are forced to use for some other reasons,
the amount of use should be restricted within a range as small as
possible so that false-twist efficiency is not adversely affected.

[0314]Polylactic acid preferably has a relative viscosity (ηrel) of
2.7 to 3.9, because an excellent false-twist yarn may be obtained, or
decrease of the tensile strength is suppressed to be minimum to decrease
break of yarns during the false-twist process in this viscosity range.

[0315]The Sn content in polylactic acid is preferably 0 or 30 ppm or less.
While the Sn based catalyst is used as a polymerization catalyst of
polylactic acid, an Sn content of 30 ppm or less permits decrease of the
tensile strength to be suppressed to its minimum besides decreasing the
incidence of break of yarns in the false-twist process.

[0316]Although polylactic acid without the properties as described above
or common resins other than polylactic acid may be used as starting
materials in the false-twist yarn, the material is preferably a
biodegradable resin such as an aliphatic polyester for manufacturing a
biodegradable false-twist yarn.

[0317]The false-twist yarn preferably has a tensile strength of 2.5
cN/dtex or more, because incidence of break of yarns and fluffs decrease
in the post-processing such as weave and knit process when the tensile
strength falls within the range above.

[0318]The false-twist yarn preferably has a contraction ratio in boiling
water of 5% or more from the view point of preventing wrinkles from
generating. The contraction ratio in boiling water of 5% or more can
prevent wrinkles from generating when fabrics are subjected to dyeing
process.

[0319]The contraction ratio in boiling water is preferably 15% or less
when the strength of the yarn is emphasized. The tensile strength and
tear strength may be secured without largely changing dimensions and mass
per unit area of the fabric when contraction ratio in boiling water is
15% or less.

[0320]A contraction ratio in boiling water of 5 to 15% is preferable for
satisfying both prevention of wrinkles and retention of strength.

[0321]The false-twist yarn preferably has a expansion/contraction recovery
ratio of 10% or more, because the fabric is endowed with flexibility to
enable the yarns to be developed in the application fields in which
stretching properties are required. Moreover crimp characteristics of the
false-twist yarn permits fabrics having a fluffy feeling to be supplied.

[0322]Commonly available false-twisting machines may be used for
false-twist of the raw thread of the false-twist yarn comprising threads
of polylactic acid. While the false-twisting machine is classified into a
cross-belt type having a twist-rotor comprising a rubber based material,
a pin-type having a twist-rotor comprising a metal, and a friction type
for twisting with a disk, the type of the machine is not particularly
restricted.

[0323]The temperature of the plate heater for heat-setting is preferably
110 to 150° C., more preferably 120 to 140° C. Since the
melting point of polylactic acid is 170° C., molecular orientation
is not disturbed at 150° C. or less to enable the tensile strength
to be avoided from largely decreased. A sufficient heat-setting is
possible, on the other hand, at 110° C. or more to make the
expansion/contraction ratio to be high to enable a false-twist yarn
having excellent crimp characteristics to be obtained.

EXAMPLES

[0324]Additional aspects will be described in detail. While analysis
processes of the physical and chemical properties of the polymer are
described herein, those not described below have been already described.

Tensile Strength

[0325]A load was applied to the sample by hanging a (indicated linear
density×1/10) grams of weight. The sample with a length of 20 cm
was drawn at a speed of 20 cm/min using a Tensiron type tensile strength
tester, and the tensile strength was calculated from the break force
using the following equation:

tensile strength(cN/dtex)=break force/actual linear density.

Ultimate Elongation

[0326]A load was applied to the sample by hanging a (indicated linear
density×1/10) grams of weight. The sample with a chuck distance of
50 cm was drawn at a speed of 50 cm/min using an Instron type tensile
strength tester to measure the chuck distance (L) when the sample is
broken, and the ultimate elongation was calculated from the following
equation:

Ultimate elongation(%)=(L-50)/50×100.

Contraction Ratio in Boiling Water

[0327]A load was applied to the sample by hanging a (indicated linear
density×1/10) grams of weight using a round scale with a frame
circumference of 100 cm. A sub-reel with a reel number of ten was
manufactured, and the sample was immersed in water at room temperature by
loading with an (indicated linear density×1/10×20) grams
weight to measure the length of the sample eight minutes after immersion.
The sample was then taken out of water, folded twice as a figure of 8 and
immersed in boiling water for 80 minutes. The sample was again loaded
with an (indicated linear density×1/10×20) grams weight in
water to measure the length eight minutes after immersion. The
contraction ratio in boiling water was calculated by the following
equation:

[0328]A load was applied to the sample by hanging a (indicated linear
density×1/10) grams of weight. A sub-reel with a reel number often
was manufactured, and the sample was immersed in water at 20±2°
C. for 3 minutes by loading with an (indicated linear
density×1/10×20) grams weight. The length (a) of the reel was
at first measured and, after allowing to stand for two minutes by
removing the load, the length (b) of the reel was measured again to
calculate the recovery ratio from the following equation:

Expansion/contraction recovery ratio(%)=(a-b)/a×100.

Work Efficiency of False-Twist

[0329]Work efficiency of false-twist was totally evaluated by the
following criteria: [0330].circleincircle.: incidence of break of yarns
is one time/day or less among 48 spindles; [0331]∘: incidence
of break of yarns is two to five times/day among 48 spindles;
[0332]Δ: incidence of break of yarns is six to 15 times/day among
48 spindles; and [0333]x: incidence of break of yarns is 16 times/day or
more among 48 spindles.

Work Efficiency of Weaving

[0334]Work efficiency of weaving when the yarn was woven using WJL was
totally evaluated by the following criteria: [0335].circleincircle.:
incidence of break of yarns is zero time a day; [0336]∘:
incidence of break of yarns is one to two times a day; [0337]Δ:
incidence of break of yarns is three to nine times a day; and [0338]x:
incidence of break of yarns is ten times or more a day.

Feeling of Textile

[0339]Feeling of textile was totally evaluated by the following criteria:
[0340].circleincircle.: fluffy feeling of the textile is nearly
identical to the textile using regular polyester yarns;
[0341]∘: fluffy feeling of the textile is somewhat inferior
to the textile using regular polyester yarns; [0342]Δ: the textile
using the false-twist yarn has somewhat better fluffy feeling than the
textile using the original yarn; and [0343]x: there is no fluffy feeling
at all.

Example 7-1

[0344]A false-twist yarn with a tensile strength of 3.2 cN/dtex and
expansion/contract recovery ratio of 16.4% was obtained from the
polylactic acid fibers having the composition shown in Table 7-1 by
heat-setting at 130° C. using a false-twisting machine 33H-Mach
Crimper (made by Murata Machine Co.) comprising a cross-belt type twist
roller. Work efficiency of the yarn was favorable, and no break of yarns
was observed after processing of 1 ton of yarns. When a textile was woven
with a water-jet loom using this false-twist warn as a woof, fabrics
having sufficient fluffy feeling can be manufactured with substantially
no break of yarns.

Example 7-2

[0345]A false-twist yarn with a tensile strength of 2.9 cN/dtex and
expansion/contract recovery ratio of 14.8% was obtained from the
polylactic acid fibers having the composition shown in Table 7-1 by
heat-setting at 130° C. using a false-twisting machine ST-5 (made
by Mitsubishi Industrial Machine Co.) comprising a pin type twist roller.
Work efficiency of the yarn relatively was favorable, and no break of
yarns was observed after processing of 1 ton of yarns. When a textile was
woven with a water-jet loom using this false-twist warn as a woof,
fabrics having sufficient fluffy feeling can be manufactured with
substantially no break of yarns.

Comparative Example 7-1

[0346]A false-twist yarn with a tensile strength of 1.9 cN/dtex and
expansion/contract recovery ratio of 13.3% was obtained from the
polylactic acid fibers containing a large proportion of monomers using a
false-twisting machine 33H-Mach Crimper (made by Murata Machine Co.)
comprising a cross-belt type twist roller. The tensile strength was low
due to large content of the monomer, and work efficiency was considerably
poor with frequent occurrence of break of yarns when a textile was woven
using this false-twist yarn as a woof with a water-jet loom.

Example 7-3

[0347]A false-twist yarn with a tensile strength of 1.2 cN/dtex and
expansion/contraction recovery ratio of 6.7% was obtained from a
polylactic acid fiber containing a small proportion of the L-isomer as
shown in Table 7-1 using the false-twisting machine used in Comparative
Example 7-1. The false-twist yarn had a little higher contraction ratio
in boiling water and a little low work efficiency. However, break of
yarns was seldom observed when a fabric was woof using this false-twist
yarn as a woof with a water jet loom.

Example 7-4

[0348]A false-twist yarn with a tensile strength of 2.2 cN/dtex and
expansion/contraction recovery ratio of 13.1% was obtained from a
polylactic acid fiber containing branched structures as shown in Table
7-1 using the false-twisting machine used in Comparative Example 7-1.
Although work efficiency was a little poor with a few times of break of
yarns since the tensile strength is inferior to the yarns having no
branched structure in Example 7-1, the expansion/contraction recovery
ratio was as high as 10% or more. When a fabric was woven using this
false-twist yarn as a woof with a water-jet weave machine, a fluffy
fabric could be manufactured with few frequency of break of yarns.

Example 7-5

[0349]A false-twist yarn with a tensile strength of 1.6 cN/dtex and
expansion/contraction recovery ratio of 14.5% was obtained from a
polylactic acid fiber having a low relative viscosity as shown in Table
7-1 using the false-twisting machine used in Comparative Example 7-1.
Although work efficiency was a little poor with a few times of break of
yarns due to a little inferior tensile strength of this false-twist yarn
to the false-twist yarn having a favorable relative viscosity in Example
7-1, the contraction rate in boiling water was low and
expansion/con-traction recovery ratio was high. When a fabric was woven
using this false-twist yarn as a woof with a water-jet loom, a fluffy
fabric could be manufactured with few frequency of break of yarns.

Example 7-6

[0350]A false-twist yarn with a tensile strength of 2.3 cN/dtex and
expansion/contraction recovery ratio of 13.3% was obtained from a
polylactic acid fiber having a high relative viscosity as shown in Table
7-1 using the false-twisting machine used in Comparative Example 7-1.
Although work efficiency was a little poor with a few times of break of
yarns due to a little inferior tensile strength of this false-twist yarn
to the false-twist yarn having a favorable relative viscosity in Example
7-1, the contraction rate in boiling water was low and
expansion/contraction recovery ratio was high. When a fabric was woven
using this false-twist yarn as a woof with a water-jet loom, a fluffy
fabric could be manufactured with few frequency of break of yarns.

Example 7-7

[0351]A false-twist yarn with a tensile strength of 1.3 cN/dtex and
expansion/contraction recovery ratio of 12.8% was obtained from a
polylactic acid fiber containing a large amount of Sn as shown in Table
7-1 using the false-twisting machine used in Comparative Example 7-1.
Although work efficiency was a little poor with a few times of break of
yarns due to a low tensile strength of this false-twist yarn as compared
with the false-twist yarn containing a small amount of Sn in Example 7-1,
the contraction rate in boiling water was low and expansion/contraction
recovery ratio was high. When a fabric was woven using this false-twist
yarn as a woof with a water-jet loom, a fluffy fabric could be
manufactured with few frequency of break of yarns.

[0353]A highly oriented non-drawn polylactic acid fiber with a
birefringence (Δn) of 0.010 to 0.035, and tensile strength S
(cN/dtex) and ultimate elongation (%) in the range of the following
equation should be used for the false-twist yarn:

15≦S× E≦23.

[0354]Since the polylactic acid fiber is inferior in heat resistance to
other synthetic fibers, at draw and twist processing filaments are
melt-fused in the polylactic acid non-drawn yarn with a birefringence
(Δn) of less than 0.010 and S× E of less than 15 to make
processing unstable. In the polylactic acid highly oriented non-drawn
yarn with a birefringence (Δn) of exceeding 0.035 and S× E of
exceeding 23, yarns having desirable properties cannot be obtained due to
too high orientation.

[0355]The heater temperature for simultaneous draw-and-twist processing is
required to be 110° C. or more. A temperature of less than
110° C. fails in obtaining a false-twist yarn having sufficient
properties.

[0356]The draw magnification factor in the simultaneous draw-and-twist
processing should be 1.3 to 1.8. Satisfactory properties cannot be
obtained at a factor of less than 1.3, while a factor of exceeding 1.8
arises break of yarns to fail in practical production.

[0357]While other polymers may be used together, a biodegradable polymer
material should be used for manufacturing a biodegradable false-twist
yarn.

EXAMPLES

Polymerization of Polymer

[0358]Polylactic acid was synthesized by the conventional process using
L-lactide and D-lactide as starting materials and tin octylate as a
polymerization catalyst. For comparison, polylactic acid was also
synthesized by adding 0.1 mol % of trimellitic acid as a cross-link
agent. The polymer obtained was further subjected to solid sate
polymerization at 135° C. to reduce the content of residual
monomers. However, solid state polymerization was omitted in a part of
the samples for comparative purposes.

Examples 8-1 to 8-4, Comparative Examples 8-1 to 8-10

[0359]Each polylactic acid was melted at a predetermined temperature and
spun from nozzle holes with a diameter of 0.3 mm. After reeling at a
spinning speed of 3800 m/min, the filaments were simultaneously drawn and
false-twisted to produce a false-twist yarn with a linear density of 84
dtex/24f. The simultaneous draw-and-false twist machine used was 33H mach
Crimper made by Murata Machine Co.

[0360]As shown in the date of the examples in Tables 8-1 to 8-4, the
false-twist yarns produced under the conditions had splendid properties.
On the contrary, as shown in the comparative examples 8-1 to 8-7, the
false twist yarns having sufficient properties could not obtained from
the non-drawn yarns with Δn, S and E out of our range.

[0362]The polylactic acid filament nonwoven fabric known in the art
include a filament nonwoven fabric having no core-and-sheath structure in
which a polymer prepared by cross-linking a polybutylene succinate
polymer synthesized from 1,4-butanediol and succinic acid with urethane
bonds is blended with polylactic acid as a binder resin. However, this
polymer composition has so poor compatibility that a filament nonwoven
fabric having a sufficient tensile strength cannot be obtained.

[0363]We strictly surveyed the properties of the polylactic acid as a
starting material of the textile, and invented a polylactic acid filament
nonwoven fabric having physical properties such as tensile strength and
expansion ratio comparable to those of polyester, nylon and polypropylene
fibers, by using polylactic acid with selected properties and having a
core-and-sheath structure.

[0364]In a first aspect, we provide a polylactic acid filament nonwoven
fabric mainly comprising polylactic acid (PLA) and having a
core-and-sheath structure, wherein the core to sheath ratio is 1:1 to 5:1
in area ratio, and the sheath component comprises polylactic acid having
a lower melting point than the core component, or the sheath component
comprises a blend of polylactic acid and other biodegradable polymers
having a lower melting point than polylactic acid.

[0365]In a second aspect, we provide a filament nonwoven fabric having a
core-and-sheath structure, wherein (a) the core component has a linear
structure with a relative viscosity of 2.5 to 3.5 and Sn content of 0 or
30 ppm or less, and polylactic acid contains 98 mol % or more of the
L-isomer, and (b) the sheath component has a linear structure with a
relative viscosity of 2.5 to 3.5 and Sn content of 0 or 30 ppm or less,
and comprises polylactic acid with 96 mol % or less of the L-isomer and
the core to sheath ratio of 1:1 to 5:1 in area ratio.

[0366]In a third aspect, we provide a filament nonwoven fabric having a
core-and-sheath structure, wherein (a) the core component has a linear
structure with a relative viscosity of 2.5 to 3.5 and Sn content of 0 or
30 ppm or less, and polylactic acid contains 98 mol % or more of the
L-isomer, and (b) the sheath component has a linear structure with a
relative viscosity of 2.5 to 3.5 and Sn content of 0 or 30 ppm or less,
and comprises a blend of polylactic acid with 98 mol % or more of the
L-isomer and a polymer prepared by cross-linking a polybutylene succinate
polymer synthesized from 1,4-butanediol and succinic acid with urethane
bonds, the weight ratio of polylactic acid being 50 to 90% and the core
to sheath ratio being 1:1 to 5:1 in area ratio.

[0367]In a more preferable embodiment, the polylactic acid filament
nonwoven fabric has a mean linear density of 1 to 15 dtex, mass per unit
area of 10 to 200 g/m2 and tensile strength in the longitudinal
direction of 30N or more.

[0368]The first aspect will be described first. In this aspect, polylactic
acid is used for the core, and polylactic acid having a lower melting
point than the core component or a blend of a biodegradable polymer
having a lower melting point than the polylactic acid with polylactic
acid is used for the sheath component. The core to sheath ratio is 1:1 to
5:1 in area ratio.

[0369]Forming the core-and-sheath structure allows polylactic acid crystal
as the core component to be fully oriented, and using polylactic acid
having a lower melting point than the core component or a blend of a
biodegradable polymer having a lower melting point than the polylactic
acid with polylactic acid gives an advantage that filaments are partially
fused with each other so that a sufficiently high tensile strength is
obtained.

[0370]The core-and-sheath fiber is required to have a core to sheath ratio
of 1:1 to 5:1. The proportion of the sheath component higher than this
range is inadequate, since the tensile strength may become insufficient
and the fiber may adhere to the hot roller to decrease work efficiency.
The proportion of the core component higher than this range is also
inadequate, since the tensile strength may decrease due to insufficient
partial fusion among the filaments or fluffs may appear in the filament
nonwoven fabric.

[0371]The second aspect will be described hereinafter. The polylactic acid
has a linear structure, or substantially has no branched structure. It
has been proposed to add a small amount of a branching agent in preparing
polylactic acid in order to improve melt viscosity and degree of
polymerization. However, we confirmed that the branched structure of the
polylactic acid com-position far more negatively affects work efficiency
of spinning as compared with conventional polyesters. In other words,
even a small proportion of the branched structure in polylactic acid
reduces the tensile strength as compared with polylactic acid having no
branched structure.

[0372]For excluding the branched structure, it is recommended to avoid use
of agents that arise the branched structure, for example three valent or
four valent alcohols and carboxylic acids, in the polymer material.
However, when such agent is forced to use for some reasons, the amount
should be restricted within a minimum essential range that does not
affect work efficiency of spinning such as break of fibers during
spinning.

[0373]The Sn content in polylactic acid is 30 ppm or less, preferably 0 or
20 ppm or less. While the Sn based catalyst is used as the polymerization
catalyst of polylactic acid, Sn content exceeding 30 ppm induces
depolymerization during spinning to extremely reduce work efficiency of
spinning.

[0374]For reducing the Sn content, the amount of Sn to be used for
polymerization may be reduced, or the polymerized chips are washed with
an appropriate solvent.

[0375]The polylactic acid has a relative viscosity (ηrel) of 2.7 to
3.9. A viscosity lower than this range reduces heat resistance of the
polymer to make it impossible to attain a sufficient tensile strength,
while the higher viscosity forces the spinning temperature to be elevated
to cause heat degradation during spinning. Therefore, the preferable
range is 2.7 to 3.0.

[0376]While polylactic acid to be used for the core component mainly
comprises L-lactic acid or D-lactic acid, L-lactide or D-lactide as a
dimer of lactic acid, or mesolactide, it is crucial that the proportion
of the L-isomer is 98 mol % or more. When the proportion of the L-isomer
is lower than 98 mol % crystal orientation during the producing process
is inhibited from advancing to deteriorate the physical properties of the
fibers obtained. The tensile strength is particularly reduced to make the
fibers practically inapplicable.

[0377]Polylactic acid to be used in the sheath component has a proportion
of the L-isomer of 96 mol % or less to allow the sheath part to have a
different melting point from the melting point of the core part. The
preferable proportion of the L-isomer is 91 to 95 mol %.

[0378]A polymer in which 10 to 50% by weight of a polymer, prepared by
cross-linking a polybutylene succinate polymer synthesized from
1,4-butanediol and succinic acid with urethane bonds and having a lower
melting point than L-lactic acid to be used for the core part, is blended
with polylactic acid is preferably used for endowing the sheath part with
fusing property. A blend ratio of exceeding 50% makes fusing property
among the filaments too high to make the nonwoven fabric to adhere on the
hot roller, thereby making work efficiency and productivity insufficient.

[0379]Various additives such as a lubricants, an oxidation stabilizer and
heat stabilizer may be added, if necessary, to the polymer in the range
not compromising the effect.

[0380]It is essential that the core-to-sheath ratio is in the range of 1:1
to 5:1 in area ratio. A larger proportion of the sheath component than
this range is inappropriate, since the tensile strength may become
insufficient or the filament nonwoven fabric may fuse the hot roller to
reduce work efficiency. A larger proportion of the core component is also
inappropriate, because filaments are not partially fused with each other
to reduce the tensile strength, or fluffs may appear in the filament
nonwoven fabric.

[0381]The filament nonwoven fabric preferably has a mean linear density of
1 to 15 dtex. When the linear density exceeds 15 dtex, cooling
performance may be poor during producing, or flexibility of the filament
nonwoven fabric may be compromised, thereby arising practical problems.
The linear density of less than 1 dtex may reduce productivity due to
frequent occurrence of break of fibers.

[0382]The third aspect will be described hereinafter. The same quality of
polylactic acid as used in the second aspect should be used in this
aspect.

[0383]The polymer for blend to be used in the sheath component is a
polymer prepared by cross-linking polybutylene succinate polymer
synthesized from 1,4-butanediol and succinic acid with urethane bonds.

[0384]For blending the polymer with polylactic acid to form a sheath
component, the required blending ratio of polylactic acid is 50 to 90% by
weight. When the proportion of polylactic acid is less than 50% by
weight, filaments are too strongly fused with each other to form a sheet,
or the filament nonwoven fabric is fused on the hot roller to reduce
productivity. When the proportion of polylactic acid exceeds 90% by
weight, on the other hand, fluffs may appear due to insufficient fusion
among the filaments with a low tensile strength to make the fabric to be
practically inapplicable.

[0385]The required core-to-sheath ratio is 1:1 to 5:1 in area ratio. A
larger proportion of the sheath component than this range is not
appropriate, since the tensile strength may become insufficient or the
filament nonwoven fabric may fuse the hot roller to reduce work
efficiency. A larger proportion of the core component is also
inappropriate since partial fusion among the filaments is not so
sufficient that the tensile strength becomes insufficient, or fluffs may
appear in the filament nonwoven fabric.

[0386]The filament nonwoven fabrics according to the three aspects as
described above preferably have a mean linear density of 1 to 15 dtex,
mass per unit area of 10 to 200 g/m2 and longitudinal tensile
strength of 30N or more. A linear density in this range permits
sufficient productivity to be obtained. A mass per unit area in this
range makes the fabric flexible, while a longitudinal tensile strength in
this range arises no troubles in respective processing steps.

[0387]The producing process of the filament nonwoven fabric comprises the
steps of, for example, dispersing the filaments while drawing by reeling
them at a reel speed of 3,000 m/min to 6,000 m/min, collecting and piling
the filaments on a moving support made of a capture wire nets, and
partially fusing the filaments on a roll at a roll temperature of 100 to
150° C. to obtain a filament nonwoven fabric.

[0388]The reel speed in this is preferable since crystal orientation
sufficiently advances to enhance work efficiency.

[0389]The roll temperature is preferably 100 to 150° C. A
temperature of higher than 150° C. is too close to the melting
point of polylactic acid of the core component that the nonwoven fabric
fuses on the roller to arise problems in productivity.

EXAMPLES

[0390]Additional examples will be described in more detail hereinafter.
The analysis method of physical and chemical properties of the polymer
will be described first. The method not described herein has been
hitherto described.

Measurement of Elongation Percentage

[0391]A sample piece with a dimension of about 5 cm×20 cm was
extracted from a sample. After attaching the sample piece to a tensile
strength tester with a chuck distance of 10 cm, the sample piece was
drawn at a draw speed of 20 cm/min to measure the load (N) at break of
the sample piece.

[0392]Spinning work efficiency was measured and evaluated as follows:

[0393]Evaluation of Productivity [0394]∘: productivity is
very excellent with good spinning ability and hot-roll passing
performance; and [0395]x: continuous production is impossible due to poor
spinning ability and hot-roll passing performance.

Examples 9-1 to 9-3

[0396]The filaments were spun at a spinning temperature of 230° C.,
reeled at a reel speed of 3,000 m/min, and captured and piled on a moving
wire capture support in Examples and Comparative Examples. The captured
filaments were processed into a filament nonwoven fabric with a mean
linear density of 2.2 dtex and mass per unit area of 30 g/m2 at a roll
temperature of 145° C.

[0397]Tables 9-1 and 9-2 show that the filament nonwoven fabric obtained
within the conditions is excellent in physical properties such as the
tensile strength and productivity.

[0398]The sample in Comparative Example 9-1 contained a larger proportion
of the L-isomer, filaments were not partially fused with each other by
hot-rolling, and a lot of fluffs were generated. The sample in
Comparative Example 9-2 having a small area ratio of the sheath part was
also absent in partial fusion among the filaments, while the sample in
Comparative Example 9-3 was, on the contrary, had a too large area ratio
of the sheath part that the nonwoven fabric fused on the hot-roll.

[0399]The sample in Comparative Example 9-4 in which a branched polymer
was used could not attain a sufficient tensile strength due to the
branched structure.

[0400]The sample in Comparative Example 9-5 containing a large amount of
residual Sn caused depolymerization during spinning to extremely reduce
spinning work efficiency.

[0401]The sample in Comparative Example 9-6 having a lower polymer
viscosity failed in obtaining a sufficient tensile strength, while the
sample in Comparative Example 9-7 having a higher polymer viscosity was
forced to elevate the spinning temperature to cause heat decomposition of
the polymer during spinning, thereby making it impossible to obtain a
filament nonwoven fabric having a sufficient tensile strength.

[0402]A polymer having a higher melting point is used in the sheath
component in Comparative Example 9-8. The filaments were not partially
fused by hot rolling due to the high melting point of the sheath
component to generate fluff in the filament spun-bond fabric, thereby
causing poor productivity. Consequently, a filament nonwoven fabric
having a sufficient tensile strength could not be obtained.

[0403]The blend ration of the polymer (trade name: Bionole, melting point
110° C.) as a sheath component, prepared by cross-linking a
polybutylene succinate polymer synthesized from 1,4-butanediol and
succinic acid by urethane bonds, is changed as shown in Table 9-3. While
there were no problems in the blend ratio within the range (Examples 9-4
and 9-5), the nonwoven fabric was fused on the hot-roll to make
production impossible in the Comparative Example 9-9 in which the
blending ratio was increased. In Comparative Example 9-8 in which the
blending ratio was reduced, on the other hand, the filaments were not
partially fused with each other to create fluffs in the nonwoven fabric.

INDUSTRIAL APPLICABILITY

[0404]We provide a textile product being excellent in work efficiency and
having excellent properties of the fiber comprising polylactic acid that
is free from practical problems for industrial production, and a process
for producing the textile product.